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110 lines
4.9 KiB
Markdown
110 lines
4.9 KiB
Markdown
# N65 NES assembler version 0.5
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This is an assembler for the Nintendo Entertainment System's 2A03
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microprocessor.
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The 2A03 is an 8-bit processor based on the MOS 6502.
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```bash
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Usage: ./n65 <infile.asm> -o outfile.nes
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```
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This is a pretty straightfoward assembler, which is currently set up
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to produce iNES formatted ROM binaries from 6502 assembly language files.
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<a href="http://irkenkitties.com/blog/2015/03/29/creating-sound-on-the-nes/">Here</a>
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is a recent blog post that goes through creating a program with this
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n65, showing the essential syntax and more. Best thing until I create
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some real documentation.
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Inside An NES cartridge there are basically some number of ROM chips
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which contain banks of either program code or character (graphics)
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data. A PROG ROM bank is generally 16KB, and a CHAR ROM bank is generally
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8KB. At least one PROG ROM bank is required, and the NES can address
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2 PROG ROM banks and 1 CHAR ROM bank without the use of a mapper.
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This assembler works on the idea of defining these banks, and allowing
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you to specify their contents. When you then assemble your output ROM
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this assembler translates the assmebly code in your your PROG banks
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into executable binary segments, and also lets you organize and address
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data in your CHAR banks. In the end it jams all these banks together
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one after another, PROG first, then CHAR, and slaps an iNES header
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on the front of it.
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It is good at knowing which addressing modes are and are not allowed for
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each instruction, and contains some examples of correct syntax.
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This assembler can now handle bankswitching if you set a
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valid mapper in the header, write more than 2 PROG banks, and then
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and write whatever bankswitching code is nessessary for the mapper
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you've chosen.
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This assembler supports symbolic labels which can be scoped. When
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writing assembly it can be easy to run out of effective names for
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labels when they are scoped globally. I have seen other assemblers
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using anonymous labels to get around this but I decided I didn't like
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that syntax very much. Instead I opted to allow opening a new scope
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where you can reuse symbol names. You can give scopes names or allow
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them to be anonymous. If you choose to name a symbol scope you can
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use a dot syntax to address any symbols that are outside your current
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scope. I should put some example code up here showing this.
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I hoped to make writing NES libraries more effective since you can basically
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namespace your symbols into your own file and not mess with anyone
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else's code. I also have also been able to use this to create C style
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structs in the memory layout, ie `sprite.x`.
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The assembler does two passes over your code, any symbols that are used
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which it hasn't seen the definition for yet return a "promise", that
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are stored for the second pass. A "promise" is a fancy name for a
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lambda/closure which promises to come up with a value later, while
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your code continues on. It then evaluates all these "promises" during
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the assembler's second pass, which fills in the missing addresses etc.
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I have used this to compile some code for the NES, and it ran correctly
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on FCEUX, got it to make some sounds, load tiles, sprites, and scrolling.
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There is an example file included (shown below) that is a modified port of
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the NES101 tutorial by Michael Martin.
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# Some new additions:
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- .byte can now handle hex and binary literals, and symbols
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- First version of Midi to NES music converter
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- added .inc directive, to include other .asm files
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- nes.asm library include file created, naming popular NES addresses
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- C Style in memory structs using .scope and .space directives
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- Explicit usage of zero page instructions with the zp suffix
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- Split the Parser into its own class
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- New MemorySpace class
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- Rewrote the Assembler class
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- Rewrote the Instruction class
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- Rewrote all directive's classes
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- Split the assembler from the commandline front-end
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- Scoped Symbol Table
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- Anonymous Scopes
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- Lower case mnemonics and hex digits
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- Ported NES101 tutor to this assembler.
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- Added msb and lsb byte selectors on address labels
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- added .org directive
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- added .dw directive
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- added .bytes directive
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- added .incbin directive
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- added .ascii directive
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- added .segment directive
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- added .scope directive
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- added .space directive
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- Invented my own iNES header directive that is JSON
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- Split the project up into separate files per class
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- Wrote some more unit tests
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- Added OptionParser for commandline opts
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- Tested a ROM with Sound output
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- Tested a ROM that changes background color
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# Some Todos:
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- Make macros that can be used interchangably inline or as a subroutine
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- Create a library for common operations, DMA, sound, etc both inline and subroutine options
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