atari2600land's Blog

A blog by atari2600land in General

Followers 10

entries
4,948
comments
2,718
views
1,808,779

Push Mr. Celery's Buttons

Entry posted by atari2600land July 23, 2016

484 views

Followers 0

So I finally fixed the problem I was having where the button press starts the game even though I wanted it to start after the button has been released and not during the button press like it was. The result was a 263 scanline count at the beginning, so I had to move around some variables to not make it do that any more. This stuff is HARD, especially if you don't know what you're doing. I see "bmi" in the code and I think "body mass index." I don't know what it stands for, but it's probably not that. That's how not used to this I am. But, with that fixed, I can continue. One of the main things I want to do is make the celery frown when you die. Let's see if I can do that...

Followers 0

1 Comment

Recommended Comments

SpiceWare 8,837

Posted July 23, 2016

From 6502 Opcodes

MNEMONIC                       HEX
BPL (Branch on PLus)           $10
BMI (Branch on MInus)          $30
BVC (Branch on oVerflow Clear) $50
BVS (Branch on oVerflow Set)   $70
BCC (Branch on Carry Clear)    $90
BCS (Branch on Carry Set)      $B0
BNE (Branch on Not Equal)      $D0
BEQ (Branch on EQual)          $F0

The minus sign is bit 7 (when looking at the bits within a byte as 76543210).

If bit 7 is on then minus is set and BMI will take the branch. TIA register INPT4 is wired into bit 7 which means you can use the BPL and BMI branches to test whether or not the firebutton is pressed.