SoCUte

SoCUte is an assembler for the Sega Saturn's SCU (System Control Unit) DSP chip, written in Rust.

The SCU-DSP is poorly documented, notoriously hard to program chip with all sorts of quirks, running at 14.3 MHz. However, it does have some horsepower to it (at least, for its day).

The goal of SoCUte is to write a more modern and portable assembler for the SCU-DSP for the purposes of homebrew and experimenting, whilst maintaining 100% compatibility with original source files. Eventually, I'm hoping to turn this into a relatively advanced macro assembler that's better than Sega's official tool.

I'm hoping that one day myself or others will be able to use this tool to unlock more power from the Saturn.

Terminology

In user-facing error messages, I refer to a single line of instructions, as visible below, as a bundle:

ad2    mov mc1,x   mov mul,p   mov mc0,y   clr a   mov a11,mc2
; the above is one single bundle

The term "bundle" is commonly used in Very-Long-Instruction-Word (VLIW) CPUs, which the SCU-DSP isn't really since the instruction word size is 32-bits. However, the ability to issue multiple instructions in a single line and the very particular and poorly documented requirements these have means that I've decided to use a bit of VLIW terminology and call these things bundles.

If your program produces error messages like this:

Illegal program: Bundle contains more than one flow control instruction

It means that you have attempted to pack more than the valid number of a particular instruction type into a single line. In this case, you would have tried to pack more than one flow control instruction (e.g. issuing two END instructions, or JMP somewhere END).

Compatibility

SoCUte removes a number of limitations from Sega's original assembler (dspasm):

• Lines may be longer than 255 characters
• Identifiers may be longer than 32 characters
• Nested IFDEF directives may be nested more than 16 levels deep

In the future I will probably add a --strict-limitations flag that will raise warnings when an incompatibility of this sort is detected.

Relaxed mode (--relaxed)

Contrary to the official documentation, there are a number of SCU-DSP programs that are written in a syntactically invalid way, but apparently once compiled on the dspasm tool.

Here's an example:

;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;
	ORG	0
;
START
;
	                                                 MOV IMP,CT0
	                                                 MOV MC0,RA0
	                                                 MOV CM,CT2

See the problem? START is supposed to be a label, and as per the documentation (SCU DSP Assembler manual pp. 1, PDF pp. 4):

• When writing labels, begin from the first column, or use a colon “:” at the end of the word (ex. LABEL: ).

Clearly, this program is malformed, but somehow it compiled anyway.

So, to combat this, SoCUte includes a --relaxed mode that will accept these invalid programs on a best-effort basis.

Information sources

• SCU User's Manual, Third edition (Sega Doc. # ST-97-R5-072694), pp. 75-173
• SCU DSP Assembler User's Manual (Sega Doc. # ST-240-A-042795)
• SCU DSP Assembler User's Manual Addendum (Sega Doc. # ST-240-A-SP1-052295)

The above are technically Sega confidential, but easily obtainable e.g. here, here

Licence

Copyright (c) 2025 Matt Young.

SoCUte is licenced under the Mozilla Public License v2.0.