BB8 wrote:
this one lets you spare the Scratch location; no speed gain
Code:
lda regCC
and #maskC ; $2A ?
sta regCC
ldx regA
txa ; A= old value
dex
stx regA
eor regA ; oldval Eor newval
and #maskAC ; $10 ?
ora ValueFlags,X
ora regCC
sta regCC
Yes, the code to do the add and subtract instructions scrub the flags at the top because of the need to capture the carry or borrow after the operation did not allow doing it below without storing an intermediate value in memory. My original dcr and inr code did the clearing afterward because I did not discover until very recently that those two instructions affected aux carry.
You are confirming that I'll have to accept having to trade six additional bytes (times about fifteen for all of the different incarnations of dcr and inr) to save using the scratch variable.
BB8 wrote:
Then of course, if memory is not an issue, you could build a DcrFlags table and do something faster:
Code:
ldx regA
dex
stx regA
lda reg_CC
and #maskC ; $2A ?
ora DcrFlag,X
sta reg_CC
And another similar table for inr. I'll have to keep this idea for later in case I want the speed.
BB8 wrote:
As for "dcr M", I'd keep the 8080's memory aligned to a page boundary, so you don't need to add the displacement for the low byte.
You are right. The memory block for the virtual 8080 is currently already page aligned. I currently forsee no need to do otherwise and will address removal of the adjustment of the lower byte later.
Login
Register
FAQ
Search
