Transfer directly to Stack?

[JohnnyRockets]

Hi all,

 

In the following "newbie" code snippet:

 

--------------

 

    processor 6502
    
    seg code
    org $F000        ; defines the code origin at $F000
    
Start:
    sei                ; disables the interrupts
    cld                ; disable the BCD decimal map mode
    ldx #$FF        ; loads the X register with #$FF
    txs                ; transfers X register to S(tack) register

 

--------------------

 

Could we have just loaded #$FF directly into the Stack register, thus saving a step/instruction?

 

 

Thank you,

 

 

 

JR

[+Stephen]

No - there's no direct loading from or saving to the stack on the 6502(7).  We can only push, or pull Accumulator values, or use the above transfer from another internal register.

[JohnnyRockets]

Thanks Stephen!

 

Now looking at this snippet:

 

---------------------------

 

    processor 6502
    
    seg code
    org $F000        ; defines the code origin at $F000
    
Start:
    sei                ; disables the interrupts
    cld                ; disable the BCD decimal map mode
    ldx #$FF        ; loads the X register with #$FF
    txs                ; transfers X register to S(tack) register
    
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Clear the Zero Page region ($00 to $FF)
; Meaning the entire TIA register space and also RAM
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    lda #0              ; A = 0
    ldx #$FF          ; X = #$FF
    sta $FF            ; make sure $FF is zeroed before the loop starts
MemLoop:
    dex                ; x--
    sta $0,X        ; store zero in A, at address $0 + X
    bne    MemLoop        ; loop until X==0 (z-flag set)
    
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;Fill ROM size to exactly 4KB
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    org $FFFC
    .word Start        ; reset vector at $FFFC (where program starts)
    .word Start        ; interrupt vector at $FFFE (unused in VCS)

 

---------------------------

 

Question:  What values are now stored in $FFFC and $FFFE?  And why?

 

My guess is  Zeros?

 

 

Thanks!

 

 

JR
   

 

[+Andrew Davie]

They both contain a 16-bit pointer to the label "Start", which is $F000

The stack lives at $FF downwards (on the '2600). Technically $1FF downwards on the 6502.

The code you post is inefficient -- use the CLEAN_START macro!

 

 

[JohnnyRockets]

Thank you Andrew, I will study this macro!

 

JR