[atari7800] Re: Why is ZP not clear in this test?


  ldx #255
.clearZP
  sta $2000,x
  dex
  bpl .clearZP
  txs
when x=255 is decremented to 254, the result will still be "negative" so the loop will exit only clearing the last byte. 

--Ken
----- Original Message ----- From: "Dennis Debro" <dennis@xxxxxxxxxxxxxxx> 
To: <atari7800@xxxxxxxxxxxxx>
Sent: Monday, August 06, 2007 3:19 PM
Subject: [atari7800] Why is ZP not clear in this test?



Hi there,

Sorry to keep coming up with questions. I must be overlooking something
with my test file. This file does character mode for 2 sets of 4 digits.
What I don't understand is when I run this via my CC2 the numbers don't
start at 0. The documentation states that ZP is shadowed at $2040 - $20FF
so to clear ZP I cleared $2000 - $20FF. Why is this area not clear?

BTW, this is my first 7800 program. As bloated as it is I actually think
I'm understanding now. Thanks for all your help.

Take care,
Dennis

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