Free NES assembler

This program reads symbolic 6502/RP2A03/RP2A07 machine code and compiles (assembles) it into a relocatable object file or into an IPS patch.

The produced object file is binary-compatible with those made with XA65.

This program was born when Bisqwit needed to have something for NES that is already accomplished for SNES by snescom.

The following mnemonics are supported:

adc, and, asl, bcc, bcs, beq, bit, bmi, bne, bpl, brk, bvc, bvs, clc, cld, cli, clv, cmp, cpx, cpy, dec, dex, dey, eor, inc, inx, iny, jmp, jsr, lda, ldx, ldy, lsr, nop, ora, pha, php, pla, plp, rol, ror, rti, rts, sbc, sec, sed, sei, sta, stx, sty, tax, tay, tsx, txa, txs, tya

All the standard addressing modes of the 6502 cpu are supported.

To be detailed later.

There are several operand prefixes that can be used to force a certain operand size/type.

• lda !$f0 would use 16-bit address instead of direct page.
• lda #<var can be used to load the lower 8 bits of an external variable.
• lda #>var can be used to load the upper 8 bits of an external variable.

Expressions are supported. These are valid code:

• bra somewhere+1
• lda #!address + $100
• ldy #$1234 + ($6C * 3)

Code, labels and data can be generated to four segments: text, data, zero and bss.
Use .text, .data, .zero and .bss respectively to select the segment.
However, only the contents of text and data segments are saved into the o65 file. Labels are saved in all segments.

Comments begin with a semicolon (;) and end with a newline.
A colon is allowed to appear in comment.

Commands are separated by newlines and colons (:).

You can use a command like *= $F200 to change where the code goes by default.
With IPS this is especially useful.
You can change the code pointer as many times as you wish, but unless you're generating an IPS file, all code must be a continuous block.

The label - can be defined for local branches backward and + for branches forward.

nescom uses GCC as a preprocessor.
You can use #ifdef, #ifndef, #define, #if, #endif and #include like in any C program. (See bugs)

Currently nescom only produces relocatable object files (O65) or non-relocatable patch files (IPS).
The O65 file format has been documented by André Fachat for the XA65 project.

This version of nescom allows you to create IPS files.
This IPS format has been extended to allow you to specify global symbols and externs to be patched later.
In the generated format:

• If the patch address is $000001, it means this entry is a dynamic symbol. What follows is an asciiz string (the symbol name), the address (3-byte lsb-first integer) and the relocation size in bytes (1 byte).
• If the patch address is $000002, it means this entry is a global symbol. What follows is an asciiz string (the symbol name) and the address (3-byte lsb-first integer) where this symbol is located in.

By default, O65 objects are linked to any free location in the ROM.
IPS files are linked to predefined locations.

With the .link statement, you can change that.

.link page $3F declares that this object should be placed into page $3F.

.link group 1 declares that this object should be placed to the same page together with all other objects that want to be linked in group 1. This is useful when you want to ensure that certain tables or routines go to the same page, even if they are not in the same compilation unit.
The actual page is determined during link time, and you can get the page by using 24-bit (@) or segment reference (^) to a symbol from those modules.

This is not completely ready for NES yet.

Nov 20 2005; 0.0.0 import from snescom-1.5.0.1.
Nov 29 2005; 0.1.0 bugfixes, now it's functional.
Dec 16 2005; 0.2.0 support for deeper "++" and "--" labels. Also support for .nop statement.
Jun 20 2006; 1.0.0 raw output file format support in snescom. Also using boost::smart_ptr.

• #included files aren't being properly preprocessed.
• memory mapping hasn't been quite designed yet.

nescom has been written by Joel Yliluoma, a.k.a. Bisqwit,
and is distributed under the terms of the General Public License (GPL).

If you happen to see this program useful for you, I'd appreciate if you tell me :) Perhaps it would motivate me to enhance the program.

The official home page of nescom is at http://iki.fi/bisqwit/source/nescom.html.
Check there for new versions.