Question about some 2600-code (shadow equates)?

[+Propane13]

In the Atari 7800 BIOS, there's some code that helps to put the the 7800 into 2600 mode, by cleaning out regs, RAM, etc.

 

(I found a copy here: http://kontek.net/pp/technical/a7800/bios7800.asm.txt)

 

I see this code, but it confuses me:

 

BIT RSYNC

BMI LF813

LF80A: LDA #$02

STA COLUBK

STA LF112

BNE LF831

LF813: BIT WSYNC

BMI LF823

 

Obviously, RSYNC ($03) and WSYNC ($02) are write-only. So, what does the read-only value pull? Is it some sort of shadow-reference to one of the collision or INPT registers?

 

Thanks,

-John K. Harvey

[DEBRO]

Obviously, RSYNC ($03) and WSYNC ($02) are write-only.  So, what does the read-only value pull?  Is it some sort of shadow-reference to one of the collision or INPT registers?

 

Thanks,

-John K. Harvey

 

Read addresses

$02 = CXP0FB ... collision between P0 and PF (D7) P0 and Ball (D6)

$03 = CXP1FB ... collision between P1 and PF (D7) P1 and Ball (D6)