Date : Fri, 12 Nov 2004 09:52:41 +0000
From : Richard_Talbot-Watkins@...
Subject: Re: SRAM
Andrew W wrote on 11/11/2004 16:37:33:
> So I assume Exile would prior to this somehow test 'sockets' 0-D(?) to
> see where the SRAM was located?
Yeah, it probably does something a bit like:
SEI \ disable interrupts to stop the OS from changing the ROM bank
LDX #15 \ start at bank 15
.loop
STX &FE30 \ enable bank X
LDY &8000 \ load Y with ?&8000
INC &8000 \ increment ?&8000
INY \ increment Y
CPY &8000 \ and compare it with the memory we tried to increment
BEQ foundram \ if equal, we have found SRAM
DEX
BPL loop \ try next bank
.notfoundram
\ if we got here, there is no SRAM
.foundram
\ if we got here, we have the SRAM bank in X, and it is still paged in
Maybe it does something a little more clever to notice 8k/12k RAM as well
(which is of course no good), and if it was written well it might even try
to skip over SRAM with a valid ROM image in it...
> I see, this is what I was beginning to suspect. So it can use virtually
> all of the 32K for the game/screen including much of below PAGE?
> Fascinating stuff :-)
Yeah, one of the first things it does is copies itself to &100 and above.
There is actually executable code over most of the stack area, the vector
table (excluding the IRQ1 vector), the OS/VDU workspace (excluding the jump
table for the BREAK key, and the byte that represents *FX200), the NMI
workspace, and right up to the beginning of the screen at either &4000 or
&6000. Not a byte wasted for sure!
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