155 lines
3.6 KiB
ReStructuredText
155 lines
3.6 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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Current status
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==============
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100% of the documented dspasm.exe features are supported, as well as
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miscellaneous undocumented features.
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scu-dsp-asm correctly assembles all of the examples in the test/
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directory in this repository, with caveats listed below.
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scp-dsp-asm has not been tested exhaustively, and the test programs do
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not cover 100% of the possible SCU DSP instructions or arguments.
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Differences that affect source code
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===================================
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M0-M3 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,M0,#$02
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DMA M1,D0,#$02
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Instead, they should be written as:
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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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``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 not consistent with what is written in the SCU manual
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(ST-97-R5-072694).
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Differences that affect code generation
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=======================================
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Spurious invalid X-bus opcode generation
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----------------------------------------
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Under seemingly-random circumstances, ``dspasm.exe`` emits the
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undefined 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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Misleading, contradictory, and completely incorrect documentation
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=================================================================
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DMA default "add mode"
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----------------------
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Contrast to what is written in the SCU manual (ST-97-R5-072694), the
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default DMA "add mode" for both dspasm.exe and scp-dsp-asm is
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``2``. E.g:
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- ``DMA`` is a synonym for ``DMA2``
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- ``DMAH`` is a synonym for ``DMAH2``
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In "Sega Developers Conference Conference Proceedings March 5-7, 1996"
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pdf page 48, printed page 3-14, slide 25:
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You need to modify your object code, either by hand or with a
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custom tool.
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This is completely incorrect, and both scu-dsp-asm and dspasm.exe will
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emit "add mode 1" via the ``DMA1`` and ``DMAH1`` mnemonics.
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DMA "add mode" and bytes
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------------------------
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All of the documentation you have read is invalid; here is the correct
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relationship between DMA "add mode" mnemonics, and the number of
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incremented bytes:
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.. list-table::
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:header-rows: 1
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:stub-columns: 1
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* - mnemonic
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- D0 → [RAM]
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- [RAM] →D0
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* - DMA0
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- 0 bytes
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- 0 bytes
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* - DMA1
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- 0 bytes
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- 2 bytes
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* - DMA2
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- 4 bytes
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- 4 bytes
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* - DMA4
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- 4 bytes
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- 8 bytes
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* - DMA8
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- 0 bytes
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- 16 bytes
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* - DMA16
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- 0 bytes
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- 32 bytes
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* - DMA32
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- 4 bytes
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- 64 bytes
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* - DMA64
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- 4 bytes
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- 128 bytes
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