[FAT32] Avoid indexed access with base in IO range on 65C816

Bank area blockwise reads and writes which wrap RAM banks in the middle of the read or write operation formerly used indirect indexed mode in $9Fxx,y to continue the operation, where .Y would start with the offset to put the first byte at $A000.

This change prevents that condition on the 65C816.

Backstory:

On the 65C816, when doing indexed reads and writes, the cycle immediately before the valid operation is a read of the indexed offset without carry.

For instance

ldy #$80
lda $9f80,y

For the lda instruction on the 65C816, just like on the original 6502, the CPU will first add .Y to the address without carry, and do a read cycle. This will read from $9f00. If the page wrapped, the CPU will do another cycle, and on this the carry of the wrap will be added to the effective address, and will do the actual read from $a000 and continue execution.

In the FAT32 code, this is done via indirect, but the effect is the same

lda #$9f
sta ptr+1
stz ptr
ldy #$80
lda (ptr),y

The lda instruction on the '816 will dereference ptr, and then do the exact same read cycle as above.

For stores, the process is similar, but slightly different

ldy #$80
lda #$69
sta $9f80,y

On the first cycle, just like above .Y will be added to the effective address without carry, and the CPU will always do a read cycle on the resulting address, even if the page did not wrap. On the next cycle, any carry will be applied and the CPU will do a write cycle on the effective address.

Indirect has the same cycle pattern after dereferencing.

Of note, an indexed write will always have that extra read cycle, otherwise in the case of a page wrap, the extra cycle will have affected the wrong address. The CPU assumes that the read doesn't have a side effect.

X16Community / x16-rom

[FAT32] Avoid indexed access with base in IO range on 65C816 #355