Re: Direct Page and Stack Pointer Question
Bryan Parkoff wrote:
65816 microprocessor allows to relocate Direct Page and Stack Pointer to
any 64K in bank zero. The problem is that instructions have only two bytes
to use Direct Page.
For example, Direct Page is at $2000. You use LDA $00 to LDA $FF that
it can only read data from $2000 to $20FF. How can you overcome the 256
bytes limit? There is no instruction to use LDA $100 or higher. The only
way is to use LDX $100 and then LDA $00,X before it can access $2100 or
higher. Remember -- it is always native mode.
It is not safe to keep code segment to $2000. It should be moved to any
absolute address like $4000. It may cause to change JMP instruction and
ohter instructions because they do not have offset unlike Direct Page and
Stack Pointer which they have offsets.
Try to think that LDA $2000 is always located at absolute address:
$2000. If code segment is at $1000 that it may be moved to $2000. LDA
$2000 can be still pointed to absolute address: $3000 with the code segment
offset so LDA $2000 is not necessary to be modified into LDA $3000.
It is sad that 65xx processors do not have code segment offset. Please
suggest what you think. If you consider to develop 32 Bits on 65xx
processors, it would be nice for Direct Page and Stack Pointer to use 32
Bits so they do not have to stay on bank zero anymore. They can be
relocated outside of 64K in bank zero.
The whole point of "zero page" addressing is to allow single-byte
addresses, and therefore speed up instruction fetch. Single-byte
addressing is, by definition, limited to 256 bytes.
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