AsmGen

Abstract

AsmGen - 6502 assembly code generator for sprites, fonts, and images for Apple ][ hi-res and Atari 8-bit computers

This program creates 6502 (or 65c02) code for several tasks, including a high speed font rendering engine and a sprite compiler. The font rendered can plot font glyphs (or game tiles) much faster than normal tile drawing routines. The sprite complier (based on HiSprite) hardcodes sprite data in unrolled loops for each shifted shape. By removing image lookups and loops, sprites can be drawn much faster than otherwise possible.

Other utilities include generating code for reporting screen damage, screen clearing, screen scrolling, and generation for all the row/column lookup tables for each supported bitmap mode.

Installation

The autogenerator code is written in Python and should run on any platform that can use python. It requires:

• python 3.6 (and above)
• numpy
• pypng

the latter two of which will be installed automatically when using the Python package manager:

pip install asmgen

Starting with version 2.0, python 2 support has been dropped.

Installing From Source

To contribute bug fixes or enhancements, it would be useful to get an account on github and clone the source to your own account. You can then send me pull requests for any modifications you would like to see included.

If you just want to clone the source to look at it, use:

git clone https://github.com/robmcmullen/asmgen.git

Using the Code Generator

The code generator is a command line tool that creates text files that can be used as source code for assemblers. Its general usage is:

asmgen.py [OPTIONS] -o ROOTNAME

which will create one file called ''ROOTNAME-driver.s'' and one or more files containing generated code as specified by ''OPTIONS''. This driver file is a convenience that only exists to include all the other generated files. This means that your build process only needs to include ROOTNAME-driver.s instead of needing to be updated if you add aditional code generation options.

Assembler Support

Code generation is supported for the following assemblers:

• cc65 (the default)
• MAC/65
• Merlin (from ksherlock)

Contributors welcome for other assemblers, like MADS, dasm, xasm, etc.

Code Generation Capabilities

• Transposed font code generator
• Sprite compiler
• Screen clearing
• Screen vertical scrolling
• Hi-res row lookup table generation

Experimental (and unsupported)

• Font compiler (slower than the transposed font code)
• Run-length encoding of images
• Merge two hi-res images, switching images at specified scan line

Transposed Font

To generate source code for a fast font renderer, you will need a binary font file. The code generator will embed the binary data as .byte directives (or equivalent for the chosen assembler) because the transposition requires reordering of the data. The usage is:

asmgen.py -f FONTFILENAME -o ROOTNAME

It will create two source files, the ROOTNAME-driver.s file described above, and the file of interest called ROOTNAME-fastfont.s which contains the generated code and data for use of the transposed font. The entry point look like this:

FASTFONT_H1 ; A = character, X = column, Y = row; A is clobbered, X&Y are not
        pha
        lda FASTFONT_H1_JMP_HI,y
        sta FASTFONT_H1_JMP+2
        lda FASTFONT_H1_JMP_LO,y
        sta FASTFONT_H1_JMP+1
        sty scratch_0
        pla
        tay
FASTFONT_H1_JMP
        jmp $ffff

FASTFONT_H1_JMP is a jump table, broken into high and low bytes.

The input is described in the comment: the glyph in the accumulator, column value (0-39) in the X register, and row value (0-23) in the Y register. Note that no error checking is done here, so it will happily trash data in some unintended part of RAM if you pass it values that are out of range.

Use it like this:

lda #65  ; ASCII character 'A'
ldx #20  ; column 20 (counting from zero)
ldy #5   ; row 5 (counting from zero)
jsr FASTFONT_H1

Fork me!

The source is free and open, and lives on github, so clone and enhance at will!

History

This program started life as a friendly fork of Quinn Dunki's HiSprite that she presented at KansasFest 2017.

Her conclusion was that it was not that practical for the Apple II because of the limited memory and the number of sprite shifts needed. I wanted to port this to the Atari because it would require fewer shifted shapes in most of the graphics modes (e.g. only 4 shifted shapes for the commonly used ANTIC modes 13 and 14) She told me that she was unlikely to continue work on sprite compiling but encouraged me to continue development.

Additional Credits

The sample font is modified from Michael Pohoreski's excellent tutorial on Apple II fonts.

License

AsmGen, the 6502 code generator sponsored by the Player/Missile Podcast Copyright (c) 2017-2021 Rob McMullen

This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.

Generated Code License

While the code for AsmGen itself is licensed under the MPL 2.0, the code it produces is licensed under the the Creative Commons Attribution 4.0 International (CC BY 4.0), so you are free to use the generated code for commercial or non-commercial purposes.