Re: [Tuxnes-devel] more help w/ asm

[Jim U. <ji...@3e...>]

At 10:28am on 2001 April 12, Rigel did write:

> heads up here.  I don't understand the implementation in table.x86.  So
> on all these listed instructions I set carry flag if there was a carrry
> (and clear it otherwise)?

Yes.  What trouble are you having with the implementation?
When carry is set, ebp is -1.  When carry is clear, ebp is 0.
For instructions which may carry, the sequence is:

addl -1, %ebp                 # set i386 carry if %ebp was set
<instruction which may carry> # set i386 carry according to 6502 instruction
sbbl %ebp, %ebp               # set %ebp to -1 on carry, else clear

Like I said, check out SEC, CLC and ROR - Accumulator, these are the
simplest examples.
 
> Yes I am trying to port the sucker to mips.  Wouldn't it be best though,
> to write the linkage (x86.S) in C?

There are a few things x86.S does:

1) acts as a call gate between asm and C -- e.g. most of the memory mappers.
   This needs to stay in assembly by definition.
2) Brings the global CLOCK, CPF, CTNI, etc. variables back in sync with the
   in-register versions -- e.g. in the INPUT:, OUTPUT:, NMI: routines, and in
   a couple mappers (mmc1, mmc3, aorom to be specific).
3) Distinguishes between IRQ and NMI in routine NMI:.  Then it sets up the
   6502 for interrupt (push return address, php, setup flags).
4) Handles self-modifying code.  Also must stay in assembly.

You can rewrite 2) and 3) in high-level code.  

** SPOILERS AHEAD ;) **

For example, the OUTPUT: routine looks like this in C:

void output_shim(int addr, int val)
{
    int cesi = ESI;
    cesi = cesi - CTNI + CLOCK;
    CTNI = ESI;
    if (cesi - CPF >= 0) { cesi -= CPF; }
    CLOCK = cesi;
    output(addr,val);
    ESI = CTNI;
}

NMI: has extra logic to handle 3)--specifically, the distinguishing between
IRQ and NMI.  Interpreter cores should implement the 6502 interrupt setup
themselves, so this can be ignored.

This is required for a C-based core.  However, there's no reason to rewrite
these in C if you're using an assembly language core, since the linkage
needs to stay anyway.

-- 
"But if food is so good for you, how come the body
keeps trying to get rid of it?" -- breatharian.com
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