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scu-dsp-asm
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scu-dsp-asm is an assembler for SCU DSP mnemonics, using SEGA syntax.
Usage:
.. code::
scu-dsp-asm input-file.asm output-file.bin
If no arguments are given, scu-dsp-asm starts a basic REPL.
Wherever possible, scu-dsp-asm attempts to preserve compatibility with
the same source code used with the SEGA-authored ``dspasm.exe``,
**except** in the cases where ``dspasm.exe`` behavior is considered a
bug.
Current status
==============
100% of the documented dspasm.exe features are supported, as well as
miscellaneous undocumented features.
scu-dsp-asm correctly assembles all of the examples in the test/
directory in this repository, with caveats listed below.
scp-dsp-asm has not been tested exhaustively, and the test programs do
not cover 100% of the possible SCU DSP instructions or arguments.
Differences that affect source code
===================================
MC0-MC3 must not appear in in DMA source/destination arguments
--------------------------------------------------------------
For example, the following are not legal:
.. code::
DMA D0,MC0,#$02
DMA MC1,D0,#$02
Instead, they should be written as:
.. code::
DMA D0,M0,#$02
DMA M1,D0,#$02
``dspasm.exe`` generates the same code given either the former or
latter example as input. scu-dsp-asm, however, rejects the former
example as invalid: it misleads what the result of the operation is.
This change is consistent with what is written in the SCU manual
(ST-97-R5-072694), but is inconsistent with SEGA's SCU DSP examples.
Differences that affect code generation
=======================================
DMA default "add mode"
----------------------
``dspasm.exe`` has a bug where, despite the SCU manual
(ST-97-R5-072694) claiming that the default DMA "add mode" is ``1``,
the generated code is add mode ``2``.
This bug is documented in "Sega Developers Conference Conference
Proceedings March 5 7, 1996" on pdf page 48, printed page 3-14,
slide 25. However, Sega of America did not fully understand the nature
of the bug: patching ``dspasm.exe`` output is not necessary, because
``dspasm.exe`` does in fact generate the correct code for the ``DMA1``
mnemonic.
In short:
- ``dspasm.exe`` assembles ``DMA`` as the equivalent of the ``DMA2``
mnemonic
- scu-dsp-asm assembles ``DMA`` as the equivalent of the ``DMA1``
mnemonic, which is the correct behavior as originally intended by
SEGA.
Spurious invalid X-bus opcodes generation
-----------------------------------------
Under seemly-random circumstances, ``dspasm.exe`` emits the undefined
X-bus opcode ``001``. Given:
.. code::
MOV M3,A MOV 1,PL
The ``dspasm.exe``-generated code is:
.. code::
00000000100001101101010100000001
^
However, the correct code should be:
.. code::
00000000000001101101010100000001
^
This is particularly interesting because the example mnemonics did not
contain any X-bus operation to start with.
Motorola S-record output is not supported
-----------------------------------------
Instead, scu-dsp-asm currently emits a raw binary file that contains
fully assembled SCU DSP code.