65816 Target Assembler in Forth

This page describes the ANSI Forth 65816 target assembler, which I wrote with the intention of using it to build the Kestrel's own Forth ROM and kernel ROM images. I was originally planning on writing them using ACME, but determined using Forth for this purpose is better due to the following reason: I'm that much closer to getting Kestrel Forth to compile itself. Otherwise, I'd have to spend the time to write the target assembler later on anyway, which would mean that I'd have to write all the guts of Forth twice! Doing the Forth-based assembler now will allow me to skip this step. (I otherwise highly recommend ACME -- it's one of my favorite assemblers!)

Features

• Supports target images up to 64KiB (possibly larger, though untested).
• Supports assembling to arbitrary locations within an image via the *! word, a convenient setter for the &* variable.
• The current location is queried via the *@ word, a convenient getter for the &* variable.
• Image origin, as seen by the target CPU, is set with the tbase variable.
• Image origin, as seen by the host environment, is set with the hbase variable.
• Assembling to multiple images concurrently is supported by swapping tbase, hbase, and &* variables as appropriate.
• Relies on external file save software, so the programmer isn't locked into a few, select formats.

Source Code

The file is so small that I'm not even going to bother putting it into a darcs repository. You can access it here. Example source files can be found towards the bottom of this page.

Addressing Mode Syntax

This assembler is an RPN assembler -- meaning that operands come before the operation. This is typical for most Forth assemblers. However, since I'm relying exclusively on the Forth parser and not writing my own, distinguishing $1234 as an absolute versus an immediate value needs to be explained, as it doesn't follow recommended Western Design Center syntax guidelines.

Word	Addressing Mode
#b,	8-bit Immediate
#w,	16-bit Immediate
#,	Immediate; size is the same as the last #b, or #w, used.
dp,	Direct Page
dp,x,	Direct Page, Indexed by X
dp,y,	Direct Page, Indexed by Y (only for STX)
(dp),	Direct Page Indirect
(,x),	Direct Page Indexed Indirect
(),y,	Direct Page Indirect Indexed
[dp],	Direct Page Indirect Long
[],y,	Direct Page Indirect Indexed Long
$,	Absolute (16-bit pointer)
$,x,	Absolute Indexed (16-bit pointer)
$,y,	Absolute Indexed (16-bit pointer)
$l,	Long Absolute (24-bit pointer)
$l,x,	Long Absolute Indexed (24-bit pointer)
,s,	Stack relative
(,s),y,	Stack relative Indirect Indexed (16-bit pointer)

Examples

Platform	Description	Format
Commodore 128	Fill text screen with a given byte	TXT
Commodore 64	Commodore 64 BASIC header	TXT
Kestrel 1r1	Cold-boot and Vector Table	TXT