crackers@xxxxxxxx wrote:
> lda blocks,x ;I'm still a little fuzzy on what exactly
> sta TEMP ;this part here is doing and how
> lda blocks+1,x ;it's doing it, but I guess it'll be in
> sta TEMP+1 ;my 6502 book.
All you are doing is getting the full 16 bit address of the particular
"block" of data you want into a 16 bit variable (low order byte/high
order byte) in the zero page. Once the address is loaded, you then read
the data using this variable address, rather than having to hardcode
each of the addresses somehow...
> ldx #$09 ;number of bytes in a block (+1)
> load lda (TEMP),x
> sta BLOCK,x
> dex
> txa
> bne load
You don't need the TXA above. And you need to use Y instead of X for
this particular loop because:
LDA (TEMP),X ;is illegal (may actually assemble without an error
message ;as...
;LDA (TEMP,X) ;which is different than what you want and would
produce ;goofy results)
;should instead be:
;LDA (TEMP),Y
X and Y are interchangeable for straight indexed reads/writes, but when
you are stretching out to indirect "pointers" they are used slightly
differently.
Also, you should set the index register to one less than the number of
items you wish to move, and end the loop with BPL, so that you don't
move a byte beyond your string, and DO move the byte at BLOCK,0. That
is, your 9 items would be indexed numbered as 0-through-8, so set up
your index to move those.
And you can save space by only using the low-order bytes in a table like
this. Assuming you can arrange your code and data so that all the data
from these blocks is in the same memory page (use an ALIGN pseudo-op to
force it, if you have enough ROM available) you could do this:
;at program init
lda #>blockPage
sta blockPtr + 1 ;preload the page of all your blocks
;later, during program execution
ldx VARIABLE ;skip the BCS by not allowing VARIABLE to
lda blocks,x ; exceed the legal limit
sta blockPtr ;store the low order byte of the desired block
ldy #8 ;index on 0-through-8
load
lda (blockPtr),y ;read
sta BLOCK,y ;write
dey ;next item in string
bpl load ;loop until index 0 has been done
blocks .byte #<block0,#<block1, etc. ;.byte instead of .word
This is some real important junk and you're definitely on the right
track. You're going to be very glad when you work out the kinks and
have the mechanics of this particular stuff all squared away in your
mind.
--
mor@xxxxxxx
http://www.crl.com/~mor/
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