2 Quick Newb ASM Questions

[John Russell Ernest]

I'm just getting started learning 6502, and have 2 questions I need some help on. I'm using the Yars' Revenge source code and the 2600 Programming Guide to learn right now.

 

1st question:

 

Here's a few lines in the code that confuse me:

 

.setYarColorValues

lda #DK_BLUE+12

sta yarColor ; save color for Yar

sta COLUP1 ; set color for Yar sprite

lda #BROWN+14

sta reservedYarColor

lda #<player1Score

sta playerScorePtr

lda #<Qotile

sta qotileGraphicPtrs

lda #>Qotile

sta qotileGraphicPtrs+1

lda #NEUTRAL_ZONE_MASK

sta neutralZoneMask

sta reserveNeutralZoneMask

ldx #4

 

What do the > / < mean for lda and when is it appropriate to use them? I know the # is immediate addressing. The qotile in this case is the qotile graphic:

 

 

Qotile

.byte $00 ; |........|

.byte $0F ; |....XXXX|

.byte $1B ; |...XX.XX|

.byte $33 ; |..XX..XX|

.byte $E3 ; |XXX...XX|

.byte $FF ; |XXXXXXXX|

.byte $E3 ; |XXX...XX|

.byte $33 ; |..XX..XX|

.byte $1B ; |...XX.XX|

.byte $0F ; |....XXXX|

.byte $00 ; |........|

 

 

2nd question:

 

On conditional branch statements (bmi, bcc, etc) does the code continue where it was before after execution of the branch (just making sure I understand branching vs. jumping)?

 

Example:

 

 

StartSwirlAnimation

 

IF COMPILE_VERSION = NTSC

 

lda #>SwirlSprites

sta qotileGraphicPtrs+1

 

ENDIF

 

lda #<SwirlSprites

sta qotileGraphicPtrs

lda #SWIRL_TRIPLE_FREQ

bit gameState

beq .jmpToCheckUpdateQotileMissile; branch if Swirl not firing at triple freq

lda gameTimer ; get current game timer

ror ; shift D0 to carry

bcc .jmpToCheckUpdateQotileMissile; branch if this is an even frame

ror

bcs CheckToLaunchSwirl ; branch if D1 of game timer set

.jmpToCheckUpdateQotileMissile

jmp .checkToUpdateQotileMissile

 

On the above bcc I bolded, does execution continue to the next ror on the line below after .jmpToCheckUpdateQotileMissile is called?

[+SpiceWare]

1. The < and > extract a specific byte of a 2-byte value.

 

Using an example 2-byte value of $FF10

 

< extracts the LSB (Least Significant Byte), or $10

> extracts the MSB (Most Significant Byte), or $FF

 

 

2. The difference between Branching and Jumping is that Branching is conditional - the branch may, or may not, be taken depending on the condition specified by the 2 letters after the B (BCC = branch if carry clear, BCS = branch if carry set, BPL = branch if number is positive, etc). If the branch is taken, it's a one-way trip, just like it is for Jumping. JSR (Jump SubRoutine) is the command that returns to continue the following code. The return is triggered by RTS (ReTurn from Subroutine).

Edited May 2, 2012 by SpiceWare