8086 Assembler for DOS

About

"8086 Assembler for DOS" (or ASM.COM for short) is a small 16-bit DOS-based two-pass self-hosting assembler for the x86 assembly language. -- It was created by Stephen Duffy in 2001. In December 2020 Robert Riebisch started maintaining his own fork at https://github.com/bttrx/8086-assembler/.

Highlights

• Supports the Intel i8086 instruction set.
• Supports the EQU pseudo-op to define constants.
• Directly generates .COM files. (No separate linker required.)
• 10 KiB small. (6 KiB after UPX-ing.)
• Free and open-source software.

Limitations

• No support for .EXE or .OBJ files.
• No support for FPU instructions.
• No support for i80186 (or higher) instructions.
• No support for declaring uninitialized data.
• No support for macros.
• No support for including other source files.
• No support for listings.
• Very limited support for expressions, i.e., only something like mov al,[bx+di] and mov ax,word ptr[codestore+2] works.
• Supports only DOS line endings (CRLF) in source file.
• Maximum line length in source file is 78 characters.
• ...

Known Bugs

1. dec r/m8, dec r/m16, and inc r/m8 instructions are encoded incorrectly. This also applies to the byte ptr or word ptr variants of div, idiv, imul, mul, neg, not, and push.
2. Better avoid whitespace around the comma between instruction operands.
3. lds and les instructions always (?) give errors. -- Reported by Japheth.
4. Neither duplicate symbols nor non-existent symbols are recognized. -- Reported by Japheth.

System Requirements

• Intel i8086/88 microprocessor (or compatibles)
• ~256 KiB total RAM
• ~20 KiB disk space
• Microsoft MS-DOS 2.0 (or compatibles)

Usage

Type at the command prompt:

asm.com infile.a

This will generate infile.com.

On success, ASM.COM prints "NNN bytes written" and exits to DOS with errorlevel 0. On error, ASM.COM prints a relevant error message and exits with errorlevel<=200 for source code errors or >200 for all other errors. For details see ASM.A file.

Language

Syntax

label:    instruction operands        ; comment

See ASM.A file for real-world examples until this paragraph has been written.

Register Names

Register names in alphabetical order, grouped by first letter.

• A ah, al, ax
• B bh, bl, bp, bx
• C ch, cl, cs, cx
• D dh, di, dl, ds, dx
• E es
• S si, sp, ss

Conventional Instructions

Conventional instructions in alphabetical order, grouped by first letter.

• A aaa, aad, aam, aas, adc, add, and
• C call, cbw, clc, cld, cli, cmc, cmp, cmpsb, cmpsw, cs:, cwd
• D daa, das, dec, div, ds:
• E es:
• H hlt
• I idiv, imul, in, inc, int, into, iret
• J ja, jae, jb, jbe, jc, jcxz, je, jg, jge, jl, jle, jmp, jna, jnae, jnb, jnbe, jnc, jne, jng, jnge, jnl, jnle, jno, jnp, jns, jnz, jo, jp, jpe, jpo, js, jz
• L lahf, lds, lea, les, lock, lodsb, lodsw, loop, loope, loopne, loopnz, loopz
• M mov, movsb, movsw, mul
• N neg, nop, not
• O: or, out
• P: pop, popf, push, pushf
• R rcl, rcr, rep, repe, repne, repnz, repz, ret, retf, rol, ror
• S sahf, sar, sbb, scasb, scasw, shl, shr, ss:, stc, std, sti, stosb, stosw, sub
• T test
• W wait
• X xchg, xlat, xor

Notes:

1. Prefix instructions lock and repCC have to be on its own line.
2. You can use int 3 to emit the special int3 instruction.
3. rep is an alias to repe.

For a detailed description of each instruction, see, e.g., Complete 8086 instruction set, NASM 2.05 based x86 Instruction Reference, or NASM 2.05 based x86 Instruction Reference (different layout).

Special Instructions (Pseudo-ops)

• byte Use with ptr to specify a variable as a byte (8 bits).
• db Declare initialized byte (8 bits).
• dw Declare initialized word (2 bytes).
• dword Use with ptr to specify a variable as a doubleword (4 bytes).
• equ (or =) Define constant.
• ptr Use with byte, dword, qword, or word to specify the data type of a variable.
• qword Use with ptr to specify a variable as a quadword (8 bytes).
• word Use with ptr to specify a variable as a word (2 bytes).

ASM.COM always sets a program's origin address to 100h. The pseudo-op org, known from other assemblers, e.g., NASM, is not supported.

Example (Short)

File hello.a:

; print message to stdout
        mov ah,9
        mov dx,msg
        int 21h

; return to DOS
        ret

; message string
msg:
        db 'hello, world',13,10,'$'

Type to build:

asm hello.a

While generating HELLO.COM 8086 Assembler will show the following:

8086 Assembler for DOS
Copyright (c) 2001 Stephen Duffy
Copyright (c) 2020-2021 Robert Riebisch
[Under GNU GPL v2]

>> Pass 1 started
12 lines processed
<< Finished successfully

>> Pass 2 started
12 lines processed
23 bytes written
<< Finished successfully

Running HELLO.COM will then produce:

hello, world

Building 8086 Assembler

To build ASM.COM no external tools are required.

Just type:

asm asm.a

That's all.

License

Copyright (c) 2001 Stephen Duffy scfduffy@gmail.com
Copyright (c) 2020-2021 Robert Riebisch rr@bttr-software.de

Usage of the works is permitted under the terms of the GNU GPL v2.
See LICENSE file for details.