Operand Format M 6809 Addressing Mode example essay topic

1.1 INTRODUCTION This is the user's reference manual for the IBM-PC hosted Motorola Freeware 8 bit cross assemblers. It details the features and capabilities of the cross assemblers, assembler syntax and directives, options, and listings. It is intended as a detailed reference and an introduction for those unfamiliar with Motorola assembler syntax and format. Those experienced with Motorola assembler products may wish to examine the file ASSEMBLER. DOC available with the cross assemblers, which briefly describes the differences between these assemblers and earlier, non-pc based versions. Assemblers are programs that process assembly language source program statements and translate them into executable machine language object files.

A programmer writes his source program using any text editor or word processor that can produce an ASCII text output. With some word processors this is known as "non document" mode. Non document mode produces a file without the non-printable embedded control characters that are used in document formatting. (Caution: assembling a file that has been formatted with embedded control characters may produce assembler errors. The solution is to convert the source file to ASCII text.) Once the source code is written, the source file is assembled by processing the file via the assembler. Cross assemblers (such as the Motorola Freeware Assemblers) allow source programs written and edited on one computer (the host) to generate executable code for another computer (the target).

The executable object file can then be downloaded and run on the target system. In this case the host is an IBM-PC or compatible and the target system is based on a Motorola 8-bit microprocessor (6800, 6801, 6803, 6805, 68 HC 05, 6809, or 68 HC 11). The assemblers are the executable programs AS. EXE where is any of 0, 1, 4, 5, 9, or 11 depending on which microprocessor you are writing code for. The details of executing the assembler programs are found in Chapter 3. The assembly language format and syntax for the various processors is very similar with slight variations due to varied programming resources (instructions, addressing modes, and registers).

These variations are explained in Appendix B. 1.2 ASSEMBLY LANGUAGE The symbolic language used to code source programs to be processed by the Assembler is called assembly language. The language is a collection of mnemonic symbols representing: operations (i. e., machine instruction mnemonics or directives to the assembler), symbolic names, operators, and special symbols. The assembly language provides mnemonic operation codes for all machine instructions in the instruction set. The instructions are defined and explained in the Programming Reference Manuals for the specific devices, available from Motorola. The assembly language also contains mnemonic directives which specify auxiliary actions to be performed by the Assembler.

These directives are not always translated into machine language. 1.3 OPERATING ENVIRONMENT These assemblers will run on any IBM-PC, XT, AT, PS-2, or true compatible. The assemblers may be run off of a floppy disk drive or they may be copied onto a hard drive for execution. DOS 2.0 or later is required. 1.4 ASSEMBLER PROCESSING The Macro Assembler is a two-pass assembler.

During the first pass, the source program is read to develop the symbol table. During the second pass, the object file is created (assembled) with reference to the table developed in pass one. It is during the second pass that the source program listing is also produced. Each source statement is processed completely before the next source statement is read. As each statement is processed, the Assembler examines the label, operation code, and operand fields. The operation code table is scanned for a match with a known opcode.

During the processing of a standard operation code mnemonic, the standard machine code is inserted into the object file. If an Assembler directive is being processed, the proper action is taken. Any errors that are detected by the Assembler are displayed before the actual line containing the error is printed. If no source listing is being produced, error messages are still displayed to indicate that the assembly process did not proceed normally. CHAPTER 2 CODING ASSEMBLY LANGUAGE PROGRAMS 2.1 INTRODUCTION Programs written in assembly language consist of a sequence of source statements. Each source statement consists of a sequence of ASCII characters ending with a carriage return.

Appendix A contains a list of the supported character set. 2.2 SOURCE STATEMENT FORMAT Each source statement may include up to four fields: a label (or " for a comment line), an operation (instruction mnemonic or assembler directive), an operand, and a comment. 2.2. 1 Label Field The label field occurs as the first field of a source statement. The label field can take one of the following forms: 1. An asterisk ( ) as the first character in the label field indicates that the rest of the source statement is a comment. Comments are ignored by the Assembler, and are printed on the source listing only for the programmer's information.

2. A white space character (blank or tab) as the first character indicates that the label field is empty. The line has no label and is not a comment. 3. A symbol character as the first character indicates that the line has a label. Symbol characters are the upper or lower case letters a- z, digits 0-9, and the special characters, period (. ), dollar sign ($), and underscore.

Symbols consist of one to 15 characters, the first of which must be alphabetic or the special characters period (.) or underscore. All characters are significant and upper and lower case letters are distinct. A symbol may occur only once in the label field. If a symbol does occur more than once in a label field, then each reference to that symbol will be flagged with an error.

With the exception of some directives, a label is assigned the value of the program counter of the first byte of the instruction or data being assembled. The value assigned to the label is absolute. Labels may optionally be ended with a colon (: ). If the colon is used it is not part of the label but merely acts to set the label off from the rest of the source line. Thus the following code fragments are equivalent: here: deca bne here here deca bne here A label may appear on a line by itself. The assembler interprets this as set the value of the label equal to the current value of the program counter.

The symbol table has room for at least 2000 symbols of length 8 characters or less. Additional characters up to 15 are permissible at the expense of decreasing the maximum number of symbols possible in the table. 2.2. 2 Operation Field The operation field occurs after the label field, and must be preceded by at least one white space character. The operation field must contain a legal opcode mnemonic or an assembler directive.

Upper case characters in this field are converted to lower case before being checked as a legal mnemonic. Thus 'nop', 'NOP', and 'NoP' are recognized as the same mnemonic. Entries in the operation field may be one of two types: Opcode. These correspond directly to the machine instructions. The operation code includes any register name associated with the instruction. These register names must not be separated from the opcode with any white space characters.

Thus 'clra' means clear accumulator A, but 'clr a' means clear memory location identified by the label 'a'. Directive. These are special operation codes known to the Assembler which control the assembly process rather than being translated into machine instructions. 2.2. 3 Operand Field The operand field's interpretation is dependent on the contents of the operation field.

The operand field, if required, must follow the operation field, and must be preceded by at least one white space character. The operand field may contain a symbol, an expression, or a combination of symbols and expressions separated by commas. The operand field of machine instructions is used to specify the addressing mode of the instruction, as well as the operand of the instruction. The following tables summarize the operand field formats for the various processor families. (NOTE: in these tables parenthesis " " signify optional elements and angle brackets " " denote an expression is inserted.

These syntax elements are present only for clarification of the format and are not inserted as part of the actual source program. All other characters are significant and must be used when required.) 2.2. 3.1 M 6800/6801 Operand Syntax The format of the operand field for M 6800/6801 instructions is: Operand Format M 6800/M 6801 Addressing Mode no operand accumulator and inherent direct, extended, or relative # immediate, X indexed Details of the M 6800/6801 addressing modes may be found in Appendix B. 2.2. 3.2 M 6804/68 HC Operand Syntax For the M 6804/68 HC 04, the following operand formats exist: Operand Format M 6804/68 HC 04 Addressing Mode no operand accumulator and inherent direct, extended, or relative # immediate bit set or clear, bit test and branch [ or ] register indirect, # move indirect Details of the M 6804/68 HC 04 addressing modes may be found in Appendix B. 2.2. 3.3 M 6805/M 68 HC 05 Operand Syntax For the M 6805/68 HC 05, the operand formats are: Operand Format M 6805/68 HC 05 Addressing Mode no operand accumulator and inherent direct, extended, or relative # immediate, X indexed, bit set or clear, , bit test and branch Details of the M 6805/68 HC 05 addressing modes may be found in Appendix B. 2.2. 3.4 M 6809 Operand Syntax For the M 6809, the following operand formats are used: Operand Format M 6809 Addressing Mode no operand accumulator and inherent direct, extended, or relative # immediate, X indexed forced extended ] extended indirect, R indexed, R forced 16-bit offset indexed [, R] indexed indirect [, R] forced 16-bit offset indexed indirect Q+ auto increment by 1 Q++ auto increment by 2 [Q++] auto increment indirect -Q auto decrement by -- Q auto decrement by 2 [ -- Q] auto decrement indirect W 1, [W 2, ...

, Wn] immediate where R is one of the registers PCR, S, U, X, or Y, and Q is one of the registers S, U, X, or Y. Wi (i = 1 to n) is one of the symbols A, B, CC, D, DP, PC, S, U, X, or Y. Details of the M 6809 addressing modes may be found in Appendix B. 2.2. 3.5 M 68 HC 11 Operand Syntax For the M 68 HC 11, the following operand formats exist: Operand Format M 68 HC 11 Addressing Mode no operand accumulator and inherent direct, extended, or relative # immediate, X indexed with X register, Y indexed with Y register bit set or clear bit test and branch The bit manipulation instruction operands are separated by spaces in this case since the HC 11 allows bit manipulation instructions on indexed addresses. Thus a ', X' or ', Y' may be added to the final two formats above to form the indexed effective address calculation. Details of the M 68 HC 11 addressing modes may be found in Appendix B. The operand fields of assembler directives are described in Chapter 4.2. 2.3. 6 Expressions. An expression is a combination of symbols, constants, algebraic operators, and parentheses.

The expression is used to specify a value which is to be used as an operand. Expressions may consist of symbols, constants, or the character ' ' (denoting the current value of the program counter) joined together by one of the operators 2.2. 3.7 Operators. The operators are the same as in c: + add - subtract multiply / divide % remainder after division & bitwise and | bitwise or ^ bitwise exclusive or Expressions are evaluated left to right and there is no provision for parenthesized expressions.

Arithmetic is carried out in signed twos complement integer precision (that's 16 bits on the IBM PC). 2.2. 3.8 Symbols. Each symbol is associated with a 16-bit integer value which is used in place of the symbol during the expression evaluation. The asterisk ( ) used in an expression as a symbol represents the current value of the location counter (the first byte of a multi-byte instruction). 2.2. 3.9 Constants. Constants represent quantities of data that do not vary in value during the execution of a program.

Constants may be presented to the assembler in one of five formats: decimal, hexadecimal, binary, or octal, or ASCII. The programmer indicates the number format to the assembler with the following prefixes: $ HEX % BINARY @ OCTAL ' ASCII Un prefixed constants are interpreted as decimal. The assembler converts all constants to binary machine code and are displayed in the assembly listing as hex. A decimal constant consists of a string of numeric digits. The value of a decimal constant must fall in the range 0-65535, inclusive. The following example shows both valid and invalid decimal constants: VALID INVALID REASON INVALID 12 123456 more than 5 digits 12345 12.3 invalid character A hexadecimal constant consists of a maximum of four characters from the set of digits (0-9) and the upper case alphabetic letters (A-F), and is preceded by a dollar sign ($).

Hexadecimal constants must be in the range $0000 to $ . The following example shows both valid and invalid hexadecimal constants: VALID INVALID REASON INVALID $12 ABCD no preceding "$" $ABCD $G 2 A invalid character $001 F $2 F 018 too many digits A binary constant consists of a maximum of 16 ones or zeros preceded by a percent sign (%). The following example shows both valid and invalid binary constants: VALID INVALID REASON INVALID %00101 1010101 missing percent %1%10011000101010111 too many digits %10100%210101 invalid digit An octal constant consists of a maximum of six numeric digits, excluding the digits 8 and 9, preceded by a commercial at-sign (@). Octal constants must be in the ranges @0 to @177777. The following example shows both valid and invalid octal constants: VALID INVALID REASON INVALID @17634 @2317234 too many digits @377 @277272 out of range @177600 @23914 invalid character A single ASCII character can be used as a constant in expressions. ASCII constants are preceded by a single quote (').

Any character, including the single quote, can be used as a character constant. The following example shows both valid and invalid character constants: VALID INVALID REASON INVALID ' 'VALID too long For the invalid case above the assembler will not indicate an error. Rather it will assemble the first character and ignore the remainder. 2.2. 4 Comment Field The last field of an Assembler source statement is the comment field.

This field is optional and is only printed on the source listing for documentation purposes. The comment field is separated from the operand field (or from the operation field if no operand is required) by at least one white space character. The comment field can contain any printable ASCII characters. 2.3 ASSEMBLER OUTPUT The Assembler output includes an optional listing of the source program and an object file which is in the Motorola S Record format. Details of the S Record format may be found in Appendix E. The Assembler will normally suppress the printing of the source listing. This condition, as well as others, can be overridden via options supplied on the command line that invoked the Assembler.

Each line of the listing contains a reference line number, the address and bytes assembled, and the original source input line. If an input line causes more than 6 bytes to be output (e.g. a long FCC directive), additional bytes (up to 64) are listed on succeeding lines with no address preceding them. The assembly listing may optionally contain a symbol table or a cross reference table of all symbols appearing in the program. These are always printed at the end of the assembly listing if either the symbol table or cross reference table options (Paragraph 4.8) are in effect. The symbol table contains the name of each symbol, along with its defined value. The cross reference table additionally contains the assembler-maintained source line number of every reference to every symbol.

The format of the cross reference table is shown in Appendix D. CHAPTER 3 RUNNING THE ASSEMBLERS 3.1 ASSEMBLER INVOCATION The Motorola Freeware Assembly programs are named as. exe where ' ' is any of 0, 1, 4, 5, 9, or 11 depending on which processor family you wish to assemble code for. For example, to generate M 6800 code run the as 0. exe program. To generate M 68 HC 05 code run the as 5. exe program, and so forth. To run the assembler enter the following command line: as file 1 (file 2... ) (- option 1 option 2... ) where file 1, file 2, etc are the names of the source files you wish to assemble. The source filenames may have extensions but the assembler does not check for any particular extension (however, do not use the. S 19 extension since that is the extension of the object file created by the assembler.

Its creation would overwrite the source file when it is written to the disk). The options are one or more of the following: l enables output listing no disables output listing (default). cre enables the cross reference table generation's enables the symbol table generation c enables cycle counting noc disables cycle counting The minus sign preceding the option should be separated from the last file name by a space. These options may also be indicated to the assembler by the use of the OPT directive in the source file. The OPT directive is described in Paragraph 4.8. The object file created is written to disk and given the name 'FILENAME. S 19' where 'FILENAME' is the name of the first source file specified on the command line.

Any errors and the optional listing (if specified) are displayed on the screen. The listing and / or error messages may be saved to a file for later examination or printing by append an i / o redirection command to the command line. On the PC i / o redirection is indicated with the greater-than symbol followed by any new or existing file name. Command line examples: The command line as 5 myfile would run the M 6805/68 HC 05 assembler on the source file 'myfile'. The object file would be written to 'myfile.'s 19' and any errors would appear on the screen. The command line as 9 test. asm nexttest.'s -l would run the M 6809 assembler on the source files 'test. asm' and 'nexttest. s'.

The object file would be written to 'test.'s 19' and any errors and the assembly listing would appear on the screen. The command line as 9 test. asm nexttest.'s -l cre's test. lst would run the M 6809 assembler on the source files 'test. asm' and 'nexttest. s'. The object file would be written to 'test.'s 19'. A listing would be created followed by a symbol table and cross reference which would all be written to the file test. lst. 3.2 ERROR MESSAGES Error diagnostic messages are placed in the listing file just before the line containing the error. The format of the error line is: Linenumber: Description of error or Linenumber: Warning -- -- Description of error Errors in pass one cause cancellation of pass two.

Warning do not cause cancellation of pass two but are indications of a possible problem. Error messages are meant to be self-explanatory. If more than one file is being assembled, the file name precedes the error: Filename, Linenumber: Description of error Some errors are classed as fatal and cause an immediate termination of the assembly. Generally this happens when a temporary file cannot be created or is lost during assembly. CHAPTER 4 ASSEMBLER DIRECTIVES 4.1 INTRODUCTION The Assembler directives are instructions to the Assembler, rather than instructions to be directly translated into object code. This chapter describes the directives that are recognized by the Freeware Assemblers.

Detailed descriptions of each directive are arranged alphabetically. The notations used in this chapter are: Parentheses denote an optional element. XYZ The names of the directives are printed in capital letters. The element names are printed in lower case and contained in angle brackets.

All elements outside of the angle brackets ' must be specified as-is. For example, the syntactical element (, ) requires the comma to be specified if the optional element is selected. The following elements are used in the subsequent descriptions: A statement's comment field A statement label An Assembler expression An Assembler expression A numeric constant A string of ASCII characters A string delimiter An Assembler option An Assembler symbol An Assembler symbol A relocatable program section M 6809 register list M 6809 register expression In the following descriptions of the various directives, the syntax, or format, of the directive is given first. This will be followed with the directive's description. 4.2 BSZ - BLOCK STORAGE OF ZEROS BSZ The BSZ directive causes the Assembler to allocate a block of bytes. Each byte is assigned the initial value of zero.

The number of bytes allocated is given by the expression in the operand field. If the expression contains symbols that are either undefined or forward referenced (i.e. the definition occurs later on in the file), or if the expression has a value of zero, an error will be generated. 4.3 EQU - EQUATE SYMBOL TO A VALUE EQU The EQU directive assigns the value of the expression in the operand field to the label. The EQU directive assigns a value other than the program counter to the label. The label cannot be redefined anywhere else in the program. The expression cannot contain any forward references or undefined symbols.

Equates with forward references are flagged with Phasing Errors. 4.4 FCB - FORM CONSTANT BYTE FCB (, , ... , ) The FCB directive may have one or more operands separated by commas. The value of each operand is truncated to eight bits, and is stored in a single byte of the object program. Multiple operands are stored in successive bytes. The operand may be a numeric constant, a character constant, a symbol, or an expression.

If multiple operands are present, one or more of them can be null (two adjacent commas), in which case a single byte of zero will be assigned for that operand. An error will occur if the upper eight bits of the evaluated operands' values are not all ones or all zeros. 4.5 FCC - FORM CONSTANT CHARACTER STRING FCC The FCC directive is used to store ASCII strings into consecutive bytes of memory. The byte storage begins at the current program counter. The label is assigned to the first byte in the string. Any of the printable ASCII characters can be contained in the string.

The string is specified between two identical delimiters which can be any printable ASCII character. The first non-blank character after the FCC directive is used as the delimiter. Example: LABEL 1 FCC, ABC, assembles ASCII ABC at location LABEL 1 4.6 FDB - FORM DOUBLE BYTE CONSTANT FDB (, , ... , ) The FDB directive may have one or more operands separated by commas. The 16-bit value corresponding to each operand is stored into two consecutive bytes of the object program. The storage begins at the current program counter.

The label is assigned to the first 16-bit value. If multiple operands are present, one or more of them can be null (two adjacent commas), in which case two bytes of zeros will be assigned for that operand. 4.7 FILL - FILL MEMORY FILL, The FILL directive causes the assembler to initialize an area of memory with a constant value. The first expression signifies the one byte value to be placed in the memory and the second expression indicates the total number of successive bytes to be initialized. The first expression must evaluate to the range 0-255. Expressions cannot contain forward references or undefined symbols.

4.8 OPT - ASSEMBLER OUTPUT OPTIONS OPT (, , ... , ) The OPT directive is used to control the format of the Assembler output. The options are specified in the operand field, separated by commas. All options have a default condition.

Some options can be initialized from the command line that invoked the Assembler, however the options contained in the source file take precedence over any entered on the command line. In the following descriptions, the parenthetical inserts specify "DEFAULT", if the option is the default condition. All options must be entered in lower case. c - Enable cycle counting in the listing. The total cycle count for that instruction will appear in the listing after the assembled bytes and before the source code. cre - Print a cross reference table at the end of the source listing. This option, if used, must be specified before the first symbol in the source program is encountered. The cross reference listing format may be found in Appendix D. l - Print the listing from this point on.

A description of the listing format can be found in Appendix D. noc - (DEFAULT) Disable cycle counting in the listing. If the "c" option was used previously in the program, this option will cause cycle counting to cease until the next "OPT c" statement. nol - (DEFAULT) Do not print the listing from this point on. An "OPT l" can re-enable listing at a later point in the program.'s - Print symbol table at end of source listing. The symbol table format can be found in Appendix D. 4.9 ORG - SET PROGRAM COUNTER TO ORIGIN ORG The ORG directive changes the program counter to the value specified by the expression in the operand field. Subsequent statements are assembled into memory locations starting with the new program counter value. If no ORG directive is encountered in a source program, the program counter is initialized to zero.

4.10 PAGE - TOP OF PAGE PAGE The PAGE directive causes the Assembler to advance the paper to the top of the next page. If no source listing is being produced, the PAGE directive will have no effect. The directive is not printed on the source listing. 4.11 RMB - RESERVE MEMORY BYTES RMB The RMB directive causes the location counter to be advanced by the value of the expression in the operand field. This directive reserves a block of memory the length of which in bytes is equal to the value of the expression. The block of memory reserved is not initialized to any given value.

This directive is commonly used to reserve a scratchpad or table area for later use. 4.12 ZMB - ZERO MEMORY BYTES (same as BSZ) ZMB The ZMB directive causes the Assembler to allocate a block of bytes. If the expression contains symbols that are either undefined or forward references, or if the expression has a value of zero, an error will be generated. APPENDIX A CHARACTER SET The character set recognized by the Freeware Assemblers is a subset of ASCII. The ASCII code is shown in the following figure. The following characters are recognized by the Assembler: 1.

The upper case letters A through Z and lower case letters a through z. 2. The digits 0 through 9.3. Five arithmetic operators: +, -, , / and % (remainder after division). 4. Three logical operators: &, |, and ^.

5. The special symbol characters: underscore, period (. ), and dollar sign ($). Only the underscore and period may be used as the first character of a symbol. 6. The characters used as prefixes for constants and addressing modes: # Immediate addressing $ Hexadecimal constant & Decimal constant @ Octal constant % Binary constant ' ASCII character constant 7. The characters used as suffixes for constants and addressing modes: , X Indexed addressing, PCR M 6809 indexed addressing, S M 6809 indexed addressing, U M 6809 indexed addressing, Y M 6809 and M 68 HC 11 indexed addressing 8.

Three separator characters: space, carriage return, and comma. 9. The character " to indicate comments. Comments may contain any printable characters from the ASCII set. 10. The special symbol backslash " " to indicate line continuation.

When the assembler encounters the line continuation character it fetches the next line and adds it to the end of the first line. This continues until a line is seen which doesn't end with a backslash or until the system maximum buffer size has been collected (typically greater or equal to 256). 11. For the M 6809 Assembler, the character " " preceding an expression to indicate extended addressing mode or 16-bit offset in indexed mode. 12. For the M 6809 Assembler, the characters used to indicate auto increment and auto decrement in the indexed mode: +, ++, -, -- .

N ^ n ~ F S 1 US /? O o DEL APPENDIX B ADDRESSING MODES B. 1 M 6800/M 6801 ADDRESSING MODES. INHERENT OR ACCUMULATOR ADDRESSING The M 6800 includes some instructions which require no operands. These instructions are self-contained and employ the inherent addressing or the accumulator addressing mode. IMMEDIATE ADDRESSING Immediate addressing refers to the use of one or two bytes of information that immediately follow the operation code in memory. Immediate addressing is indicated by preceding the operand field with the pound sign or number sign character (#).

The expression following the # will be assigned one or two bytes of storage, depending on the instruction. RELATIVE ADDRESSING Relative addressing is used by branch instructions. Branches can only be executed within the range -126 to +129 bytes relative to the first byte of the branch instruction. For this mode, the programmer specifies the branch address expression and places it in the operand field. The actual branch offset is calculated by the assembler and put into the second byte of the branch instruction. The offset is the two's complement of the difference between the location of the byte immediately following the branch instruction and the location of the destination of the branch.

Branches out of bounds are flagged as errors by the assembler. INDEXED ADDRESSING Indexed addressing is relative to the index register. The address is calculated at the time of instruction execution by adding a one-byte displacement (in the second byte of the instruction) to the current contents of the X register. Since no sign extension is performed on this one-byte displacement, the offset cannot be negative.

Indexed addressing is indicated by the characters ", X" following the expression in the operand field. The special case of ", X", without a preceding expression, is treated as "0, X". DIRECT AND EXTENDED ADDRESSING Direct and extended addressing utilize one (direct) or two (extended) bytes to contain the address of the operand. Direct addressing is limited to the first 256 bytes of memory.

Direct and extended addressing are indicated by only having an expression in the operand field. Direct addressing will be used by the Assembler whenever possible. B. 2 M 6804/M 68 HC 04 ADDRESSING MODES. IMMEDIATE ADDRESSING Immediate addressing refers to the use of one byte of information that immediately follows the operation code in memory. The expression following the # will be assigned one byte of storage. Branches can only be executed within the range -15 to +16 bytes relative to the first byte of the branch instruction. DIRECT AND EXTENDED ADDRESSING Direct and extended addressing utilize byte to contain the address of the operand.

Extended addressing concatenates the four least-significant bits of the opcode with the byte following the opcode to form a 12-bit address. Direct addressing will be used by the Assembler whenever possible. SHORT DIRECT Some opcodes allow 4 memory locations in data space ram ($80, $81, $82, and $83 to be referenced as part of the opcode. The opcode determines the data space RAM location, and the instruction is only one byte. The X and Y registers are at locations $80 and $81, respectively. An expression used with short direct addressing must not be forward referenced (that is its definition must occur before, not after this point in the file) and must equate to the range $80- $83.

BIT SET AND CLEAR In the bit set / clear addressing mode, the bit to be set or cleared is part of the opcode. The byte following the opcode specifies the direct address of the byte which will have the bit set or cleared. Any bit in the 256 byte data space memory that can be written (with the exception of the data direction registers) can be set or cleared with these two byte instructions. BIT TEST AND BRANCH The bit test and branch addressing mode is a combination of the direct addressing and relative addressing. The bit to be tested, and it condition (set or clear), is included in the opcode. The data space address of the byte to be tested is in the single byte immediately following the opcode byte and follows direct addressing rules.

The third byte is sign extended by the processor during execution to form the 12-bit relative address which is added to the program counter if the condition is true. This allows branches based on any readable bit in the data space. The branch span is -125 to +130 from the opcode address. The branch target address is used by the programmer to signify the relative offset -- the assembler calculates the offset value.

REGISTER INDIRECT In the register indirect mode, the operand is at the address in data space pointed to by the contents of one of the indirect registers, X or Y. The particular indirect register is encoded in bit 4 of the opcode by the assembler. The assembler operand syntax for register indirect is [ or ] MOVE IMMEDIATE The MVI (move immediate) instruction has its own format: mvi, # where is a direct address and is the data value to be written. MISCELLANEOUS SYNTAX ISSUES The registers in the 6804/HC 6804 are memory locations and have addresses assigned to them. The assembler has predefined a = A = $FF b = B = $80 c = C = $81 This also means that for the '04 assembler clr x is equivalent to clr since x is both a register and a memory location.

The '04 series has separate program and data spaces. There is no program memory in the range $10-$7 F. Bytes assembled into that range will go into the data space. B. 3 M 6805/68 HC 05 ADDRESSING MODES. INHERENT OR ACCUMULATOR ADDRESSING The M 6805 includes some instructions which require no operands. The address is calculated at the time of instruction execution by adding a one- or two-byte displacement to the current contents of the X register. The displacement immediately follows the operation code in memory.

If the displacement is zero, no offset is added to the index register. In this case, only the operation code resides in memory. Since no sign extension is performed on a one-byte displacement, the offset cannot be negative. Some instructions do not allow a two-byte displacement. Some instructions do not allow extended addressing.

Direct addressing will be used by the Macro Assembler whenever possible. BIT SET OR CLEAR The addressing mode used for this type of instruction is direct, although the format of the operand field is different from the direct addressing mode described above. The operand takes the form, . indicates which bit is to be set or cleared. It must be an absolute expression in the range 0-7. It is used in generating the operation code. is handled as a direct address, as described above. Since the bit manipulation address is direct, only the first 256 locations may be operated on by bit manipulation operations.

BIT TEST AND BRANCH This combines two addressing modes: direct and relative. The format of the operand is: , , . and are handled in the same manner as described above in "bit set or clear". is used to generate a relative address, as described above in "relative addressing". B. 4 M 6809 ADDRESSING MODES. INHERENT OR ACCUMULATOR ADDRESSING The M 6809 includes some instructions which require no operands. Immediate addressing is indicated by preceding the operand field with the pound sign or number sign (#) -- i. e., #. All instructions referencing the accumulator "A" or "B", or the condition code register "CC", will generate a one-byte immediate value. Also, immediate addressing used with the PSHS, PULS, PSHU, and PULU instructions generates a one-byte immediate value.

Immediate operands used in all other instructions generate a two-byte value. The register list operand does not take the form # but still generates one byte of immediate data. The form of the operand is: R 1, R 2, ... , Rn where Ri (i = 1 to n) is one of the symbols A, B, CC, D, DP, PC, S, U, X or Y. The number and type of symbols vary, depending on the specific instruction. For the instructions PSHS, PULS, PSHU, and PULU, any of the above register names may be included in the register list. The only restriction is that "U" cannot be specified with PSHU or PULU, and "S" cannot be specified with PSHS or PULS.

The one-byte immediate value assigned to the operand is calculated by the assembler and is determined by the registers specified. Each register name causes the assembler to set a bit in the immediate byte as follows: Register Bit -- - PC 7 U, S 6 Y 5 X 4 DP 3 B, D 2 A, D 1 CC 0 For the instructions EXG and TFR, exactly two of the above register names must be included in the register list. The other restriction is the size of the registers specified. For the EXG instruction, the two registers must be the same size.

For the TFR instruction, the two registers must be the same size, or the first can be a 16-bit register and the second an 8-bit register. In the case where the transfer is from a 16-bit register to an 8-bit register, the least significant 8 bits are transferred. The 8-bit registers are A, B, CC, and DP. The 16-bit registers are D, PC, S, U, X, and Y. The one-byte immediate value assigned to the operand by the assembler is determined by the register names. The most significant four bits of the immediate byte contain the value of the first register name; the least significant four bits contain the value of the second register, as shown by the following table: . Register Value (hex) D 0 X 1 Y 2 U 3 S 4 PC 5 A 8 B 9 CC A DP B RELATIVE ADDRESSING Relative addressing is used by branch instructions.

There are two forms of the branch instruction. The short branch can only be executed within the range -126 to +129 bytes relative to the first byte of the branch instruction. The long branch can execute in the full range of addressing from 0000- (hexadecimal) because a two- byte offset is calculated by the assembler and put into the operand field of the branch instruction. Direct and extended addressing are indicated by having only an expression in the operand field (i. e., ). Direct addressing will be used whenever possible. Regardless of the criteria described above, it is possible to force the Assembler to use the direct addressing mode by preceding the operand with the " " character.

These two operand forms are: . INDEXED ADDRESSING Indexed addressing is relative to one of the index registers. The general form is, R. The address is calculated at the time of instruction execution by adding the value of to the current contents of the index register. The other general form is [, R]. In this indirect form, the address is calculated at the time of instruction execution by first adding the value of to the current contents of the index register, and then retrieving the two bytes from the calculated address and address+1. This two-byte value is used as the effective address of the operand.

The allowable forms of indexed addressing are described below. In the description below, R refers to one of the index registers S, U, X, or Y. The accumulator offset mode allows one of the accumulators to be specified instead of an. Valid forms are: ., R and [, R] where is one of the accumulators A, B, or D. This form generates a one-byte operand (post-byte only). When accumulator A or B is specified, sign extension occurs prior to adding the value in the accumulator to the index register.

The valid forms for the automatic increment / decrement mode are shown below. For each row, the three entries shown are equivalent. Like the accumulator offset mode, this form generates a one-byte operand (post-byte only). The valid forms for the expression offset mode are: R, R, R [R] [, R] [, R] [, R] The " " characters force an 8- or 16-bit offset, respectively, and are described below. If no expression is specified, or if an expression with a value of zero is specified, only the postbyte of the operand is generated. If an expression with a value in the range -16 to +15 is specified without indirection, a one- byte operand is generated which contains the expression's value, as well as the index register indicator.

At execution time, the expression's value is expanded to 16 bits with sign extension before being added to the index register. All other forms will generate a post-byte, as well as either a one- or two-byte offset which contains the value of the expression. The size of the offset is determined by the type and size of the expression. Expressions with values in the range -128 to +127 generate an 8-bit offset. All other cases will result in a 16-bit offset being generated. In the case where an 8-bit offset is generated, the value is expanded to 16 bits with sign extension at execution time.

Regardless of the criteria described above, it is possible to force the Assembler to generate an 8-bit offset by preceding the operand with the " " character. If the relative address calculated is not in the range -128 to +127, or if the expression references a symbol that has not yet been defined, a two-byte offset is generated after the post-byte. A one- byte offset is generated if the relative address is in the range -128 to +127. Like the expression offset mode, a one-byte offset can be forced by preceding the operand with a " " forces a two-byte offset. A byte overflow error is generated if a one-byte offset is forced when the relative address is not in the range -12 8 to +127. The extended indirect mode has the form: Although extended indirect is a logical extension of the extended addressing mode, this mode is implemented using an encoding of the postbyte under the indexed addressing mode.

A post-byte and a two- byte offset which contains the value of the expression is generated. B. 5 M 68 HC 11 ADDRESSING MODES. PREBYTE The number of combinations of instructions and addressing modes for the 68 HC 11 is larger than that possible to be encoded in an 8-bit word (256 combinations). To expand the opcode map, certain opcodes ($18, $1 A, and $CD) cause the processor to fetch the next address to find the actual instruction. These opcodes are known as prebyte and are inserted automatically by the assembler for those instructions that require it. l In general the instructions contained in the alternate maps are those involving the Y register or addressing modes that involve the Y index register. Thus the programmer make the tradeoff between the convenience of using the second index register and the additional time and code space used by the prebyte. INHERENT OR ACCUMULATOR ADDRESSING The M 68 HC 11 includes some instructions which require no operands.

IMMEDIATE ADDRESSING Immediate addressing refers to the use of one or more bytes of information that immediately follow the operation code in memory. INDEXED ADDRESSING Indexed addressing is relative one of the index registers X or Y. The address is calculated at the time of instruction execution by adding a one-byte displacement to the current contents of the X register. If the displacement is zero, zero resides in the byte following the opcode. BIT (S) SET OR CLEAR The addressing mode used for this type of instruction is direct, although the format of the operand field is different from the direct addressing mode described above. The operand takes the form where the two expressions are separated by a blank. signifies the operand address and may be either a direct or an indexed address. When the address mode is indexed, is followed by ', R' where R is either X or Y. This allows bit manipulation instructions to operate across the complete 64 K address map. is the mask byte.

The bit (s) to be set or cleared are indicated by ones in the corresponding location (s) in the mask byte. The mask byte must be an expression in the range 0-255 and is encoded by the programmer. BIT TEST AND BRANCH This combines two addressing modes: direct or indexed and relative. The format of the operand is: where the expressions are separated by blanks. identifies the operand an may indicate either a direct or indexed address. Indexed addresses are signified with ', R' following the expression where R is either X or Y. is the mask byte. The mask byte must be an expression in the range 0-255 and is encoded by the programmer. is used to generate a relative address, as described above in "relative addressing".

APPENDIX C DIRECTIVE SUMMARY A complete description of all directives appears in Chapter 4. ASSEMBLY CONTROL ORG Origin program counter SYMBOL DEFINITION EQU Assign permanent value DATA DEFINITION / STORAGE ALLOCATION BSZ Block storage of zero; single bytes FCB Form constant byte FCC Form constant character string FDB Form constant double byte FILL Initialize a block of memory to a constant RMB Reserve memory; single bytes ZMB Zero Memory Bytes; same and BSZ LISTING CONTROL OPT c Enable cycle counting OPT cre Print cross reference table OPT l Print source listing from this point OPT nol Inhibit printing of source listing from this point OPT's Print symbol table PAGE Print subsequent statements on top of next page APPENDIX D ASSEMBLER LISTING FORMAT The Assembler listing has the following format: LINE# ADDR OBJECT CODE BYTES [ # CYCLES] SOURCE LINE The LINE# is a 4 digit decimal number printed as a reference. This reference number is used in the cross reference. The ADDR is the hex value of the address for the first byte of the object code for this instruction.

The OBJECT CODE BYTES are the assembled object code of the source line in hex. If an source line causes more than 6 bytes to be output (e.g. a long FCC directive), additional bytes (up to 64) are listed on succeeding lines with no address preceding them. The # CYCLES will only appear in the listing if the "c" option is in effect. It is enclosed in brackets which helps distinguish it from the source listing. The SOURCE LINE is reprinted exactly from the source program, including labels. The symbol table has the following format: SYMBOL ADDR The symbol is taken directly from the label field in the source program.

The ADDR is the hexadecimal address of the location referenced by the symbol. The cross reference listing has the following format: SYMBOL ADDR LOC 1 LOC 2 LOC 3... The SYMBOL and ADDR are the same as above. The indicates the start of the line reference numbers. The LOCs are the decimal line numbers of the assembler listing where the label occurs. APPENDIX E S-RECORD INFORMATION E. 1 INTRODUCTION The S-record output format encodes program and data object modules into a printable (ASCII) format.

This allows viewing of the object file with standard tools and allows display of the module while transferring from one computer to the next or during loads between a host and target. The S-record format also includes information for use in error checking to insure the integrity of data transfers. E. 2 S-RECORD CONTENT S-Records are character strings made of several fields which identify the record type, record length.