mirror of https://github.com/mgcaret/rom4x.git
71 lines
3.2 KiB
Plaintext
71 lines
3.2 KiB
Plaintext
A potential possibility is to replace Apple Slinky firmware with the RamFactor firmware.
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The ROM of the RamFactor is 8K in size, but that is not how much we'd need in the IIc.
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The layout of the ROM is two banks of 4K, selected by the low bit of $C08F+slot*$10.
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Each bank is laid out as follows, with the base at $C000
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$C000-$C0FF: Copyright message and fill, not visible in-system.
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$C100-$C7FF: Slot firmware, $100 bytes for each slot, same code in both banks
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This represents the same code assembled for each slot.
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Only the slot the card is in is visible.
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$C800-$CFFF: 2K of shared firmware space firmware, different for each bank.
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So as we see, without any modifications we need approximately 4K+256 bytes of space to
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accomodate the RamFactor code. It is likely we will need to do some bank switching which
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tends to add code, but also make some optimizations or drop features, which will remove
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code.
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There is definitely enough space in the //c firmware for it. Especially if we can drop
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the original slinky code, which uses $C400-$C4FF, $C752-$C762 (bank switch, both banks),
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and $D800-$DB58 (eff. $DBFF). $C763-$C7FF is already used by ROM 4X.
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Features that might be droppable: More than 1MB memory support.
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Steps needed:
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Produce commented RF source that assembles to a binary match for the latest firmware (1.4).
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Port to Apple //c:
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- The main issue is that we don't have the ability to run RF code at $C800.
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- Bank switch code will need to be in the C400 space, but in the alt bank that
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has the //c diagnostics, which will either need to be modified or we will have to
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use less efficient bank switching (RTS trick). We can optimize the RTS trick in this
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scenario by having an entry point table all on the same page. If we waste a few bytes
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in the alt firmware bank we can do something like:
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entry: ; expect x = function code
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lda table_hibyte
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pha
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txa
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asl
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asl ; mult by 4
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pha ; "return addr" now a multiple of 4 in jump table
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jmp swrts ; switch bank and go
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- If the RF code calls any monitor routines we may need to supply them outright
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or provide for a bank-switched call.
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- We are likely going to need a "cross-bank JSR".
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- Caller: Push callee address-1 on stack and JSR to xb_jsr
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- xb_jsr: change banks, stack has: SP->caller callee
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rework stack so that we have the following return entries
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SP->callee swrts caller
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sta $ ; into screen holes
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stx $
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tsx ; SP->caller callee, x->caller callee
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lda $103,x ; callee high
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pha ; SP->callee[h] caller callee
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lda $102,x ; callee low
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pha ; SP->callee caller callee
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lda $101,x ; caller high
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sta $103,x ; SP->callee caller callee[l]+caller[h]
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lda $100,x ; caller low
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sta $102,x ; SP->callee caller caller
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lda #swrts_hi-1
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sta $101,x
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lda #swrts_lo-1
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sta $100,x ; SP->callee swrts caller
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lda $ ; from screen holes
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ldx $
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rts ; pops callee, callee rts pops swrts, swrts pops caller
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; above code is ~44 bytes
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