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Better comment the writelog code

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git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@4822 8ced0084-cf51-0410-be5f-012b33b47a6e
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nakeee 2010-01-13 11:09:20 +00:00
parent ecdcffc7cf
commit fd40513fed
1 changed files with 18 additions and 10 deletions
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28
Source/Core/DSPCore/Src/DSPIntExtOps.cpp
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@ -23,11 +23,13 @@
// opcode). Extended opcodes do not modify program counter $pc register. // opcode). Extended opcodes do not modify program counter $pc register.
// Most of the suffixes increment or decrement one or more addressing registers // Most of the suffixes increment or decrement one or more addressing registers
// (the first four, ARx). The increment/decrement is either 1, or the corresponding // (the first four, ARx). The increment/decrement is either 1, or the
// "index" register (the second four, IXx). The addressing registers will wrap // corresponding "index" register (the second four, IXx). The addressing
// in odd ways, dictated by the corresponding wrapping register, WP0-3. // registers will wrap in odd ways, dictated by the corresponding wrapping
// register, WP0-3.
// The following should be applied as a decrement (and is applied by dsp_decrement_addr_reg): // The following should be applied as a decrement (and is applied by
// dsp_decrement_addr_reg):
// ar[i] = (ar[i] & wp[i]) == 0 ? ar[i] | wp[i] : ar[i] - 1; // ar[i] = (ar[i] & wp[i]) == 0 ? ar[i] | wp[i] : ar[i] - 1;
// I have not found the corresponding algorithms for increments yet. // I have not found the corresponding algorithms for increments yet.
// It's gotta be fairly simple though. See R3123, R3125 in Google Code. // It's gotta be fairly simple though. See R3123, R3125 in Google Code.
@ -428,9 +430,14 @@ void nop(const UDSPInstruction& opc)
} // end namespace DSPInterpeter } // end namespace DSPInterpeter
// The Writebacklog needs more commenting. It seems to be a way of writing values from the // The ext ops are calculated in parallel with the actual op. That means that
// "beginning" of the execution of an instruction, at the end of the execution. // both the main op and the ext op see the same register state as input. The
// output is simple as long as the main and ext ops don't change the same
// register. If they do the output is the bitwise or of the result of both the
// main and ext ops.
// The ext op are writing their output into the backlog which is
// being applied to the real registers after the main op was executed
void applyWriteBackLog() void applyWriteBackLog()
{ {
// always make sure to have an extra entry at the end w/ -1 to avoid // always make sure to have an extra entry at the end w/ -1 to avoid
@ -442,14 +449,15 @@ void applyWriteBackLog()
} }
} }
// This function is being called in the main op after all input regs were read
// and before it writes into any regs. This way we can always use bitwise or to
// apply the ext command output, because if the main op didn't change the value
// then 0 | ext output = ext output and if it did then bitwise or is still the
// right thing to do
void zeroWriteBackLog() void zeroWriteBackLog()
{ {
// always make sure to have an extra entry at the end w/ -1 to avoid // always make sure to have an extra entry at the end w/ -1 to avoid
// infinitive loops // infinitive loops
// What does this actually do? It just writes zeroes to registers that are
// mentioned in the write back log, without checking that the indexes aren't -1.
// Doesn't really seem sane - shouldn't it check for -1, at least?
for (int i = 0; writeBackLogIdx[i] != -1; i++) for (int i = 0; writeBackLogIdx[i] != -1; i++)
dsp_op_write_reg(writeBackLogIdx[i], 0); dsp_op_write_reg(writeBackLogIdx[i], 0);
} }