68000 Assembler - User's Manual

68000 Assembler

by Paul McKee

User's Manual

Table of Contents

1. Introduction ............................. 2

2. Source Code Format ....................... 3

2.1 Source Line Format....................... 3

2.1.1 Label Field............................ 3

2.1.2 Operation Field........................ 3

2.1.3 Operand Field.......................... 3

2.1.4 Comment Field.......................... 4

2.2 Symbols.................................. 4

2.3 Expressions.............................. 4

2.3.1 Operands in Expressions................ 4

2.3.1.1 Decimal Numbers...................... 4

2.3.1.2 Hexadecimal Numbers.................. 4

2.3.1.3 Binary Numbers....................... 5

2.3.1.4 Octal Numbers........................ 5

2.3.1.5 ASCII Constants...................... 5

2.3.2 Operators in Expressions............... 5

2.4 Addressing Mode Specifications........... 6

3. Assembly Details ......................... 7

3.1 Branch Instructions...................... 7

3.2 MOVEM Instruction........................ 7

3.3 Quick Instructions (MOVEQ, ADDQ, SUBQ)... 8

4. Assembler Directives ..................... 9

4.1 ORG - Set Origin......................... 9

4.2 Symbol Definition Directives............. 9

4.2.1 EQU - Equate Symbol.................... 9

4.2.2 SET - Set Symbol....................... 9

4.2.3 REG - Register List Symbol............. 10

4.3 Data Storage Directives.................. 10

4.3.1 DC - Define Constant................... 10

4.3.2 DCB - Define Constant Block............ 11

4.3.3 DS - Define Storage.................... 12

4.4 END - End of Source File................. 13

4.5 INCLUDE - directive...................... 13

5. Usage .................................... 14

5.1 Command Line............................. 14

5.2 Listing File Format...................... 14

5.3 Object Code File Format.................. 15

1 2

1. Introduction

The program described here, 68000 Assembler, is a basic two-

pass assembler for the 68000 and 68010 microprocessors. It

supports the complete instruction set of both processors as well

as a modest but capable set of assembler directives. The program

produces formatted listing files as well as object code files in

S-record format.

The program was written in VAX-11 C by Paul McKee during the

fall semester, 1986. The program should be portable (with some

changes) to any C language implementation that supports 32-bit

integers. 3

2. Source Code Format

2.1 Source Line Format

The input to the assembler is a file containing instruc

tions, assembler directives, and comments. Each line of the file

may be up to 256 characters long. It is recommended, however,

that the source lines be no longer that 80 characters, as this

will guarantee that the lines of the listing file do not exceed

132 characters in length. The assembler treats uppercase and

lowercase identically.

Each line of the source code consists of the following

fields:

LABEL OPERATION OPERAND,OPERAND,... COMMENT

For example,

LOOP MOVE.L (A0)+,(A1)+ Sample source line

The fields may be separated by any combination of spaces and

tabs. Except for the comment field and quoted strings, there must

be no spaces or tabs within a field.

2.1.1 Label Field

Legal labels follow the rules for forming symbol names

described in section 2.2. Labels may be distinguished in one of

two ways: (1) They may begin in column 1, or (2) they may end in

a colon, which does not become part of the label but simply

serves to mark its end. A line may consist of a label alone.

When a label is encountered in the source code, it is defined to

have a value equal to the current location counter. This symbol

may be used elsewhere is the program to refer to that location.

2.1.2 Operation Field

The operation field specifies the instruction that is to be

assembled or the assembler directive that is to be performed. A

size code (.B, .W, .L, or .S) may be appended to the operation

code if allowed, to specify Byte, Word, Long, or Short opera

tions, respectively. The operation field must not begin in the

column 1, because the operation would be confused with a label.

2.1.3 Operand Field

The operand field may or may not be required, depending on

the instruction or directive being used. If present, the field

consists of one or more comma-separated items with no intervening

spaces or tabs. (There may be spaces or tabs within an item, but

only within quoted strings.) 4

2.1.4 Comment Field

The comment field usually consists of everything on a source

line after the operand field. No special character is needed to

introduce the comment, and it may contain any characters desired.

A comment may also be inserted in the source file in another

way: An asterisk ("*") at the beginning of the line or after the

label field will cause the rest of the line to be ignored, i.e.,

treated as a comment.

2.2 Symbols

Symbols appear in the source code as labels, constants, and

operands. The first character of a symbol must be either a

letter (A-Z) or a period ("."). The remaining characters may be

letters, dollar signs ("$"), periods ("."), or underscores("_").

A symbol may be of any length, but only the first 8 characters

are significant. Remember that capitalization is ignored, so

symbols which are capitalized differently are really the same.

2.3 Expressions

An expression may be used in the source program anywhere a

number is called for. An expression consists of one or more

operands (numbers or symbols), combined with unary or binary

operators. These components are described below. The value of

the expression and intermediate values are always computed to 32

bits, with no account being made of any overflow that may occur.

(Division by zero, however, will cause an error.)

2.3.1 Operands in Expressions

An operand in an expression is either a symbol or one of the

following sorts of constants.

2.3.1.1 Decimal Numbers

A decimal number consists of a sequence of decimal digits

(0-9) of any length. A warning will be generated if the value of

the number cannot be represented in 32 bits.

2.3.1.2 Hexadecimal Numbers

A hexadecimal number consists of a dollar sign ("$") fol

lowed by a sequence of hexadecimal digits (0-9 and