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