Calcolatori Elettronici I – Commandline Assembler

68kasm68000 Assembler 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. 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.

2. Source Code Format

2.1 Source Line Format

The input to the assembler is a file containing instructions, 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 than 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 in 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 operations, respectively. The operation field must not begin in 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.)

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 ("$") followed by a sequence of hexadecimal digits (0-9 and A-F).