Detecting POKEY

Detection Code for POKEY in different locations

The following code (taken from 7800Basic, and originally adapted from GroovyBee's POKEY detection routine) checks both common POKEY locations ($450, and $4000) for the presence of a POKEY chip.

The code requires a pair of zero page locations (pokeybaselo and pokeybasehi) and another location (pokeydetected). If a POKEY chip is found, pokeydetected=0 and (pokeybaselo) will point at the POKEY memory location, through which you can access the POKEY registers indirectly.

; Pokey register relative locations, since its base may be different
; depending on the hardware.
PAUDF0   = $0    ; extra audio channels and frequencies
PAUDC0   = $1
PAUDF1   = $2
PAUDC1   = $3
PAUDF2   = $4
PAUDC2   = $5
PAUDF3   = $6
PAUDC3   = $7
PAUDCTL  = $8    ; Audio Control
PRANDOM  = $A    ; 17 bit polycounter pseudo random
PSKCTL   = $F    ; Serial Port control

detectpokeylocation
         ;XBoard/XM...
         lda #%00010000
         sta $470 ; tell XCTRL to enable the POKEY
         ldx #1
detectpokeyloop
         lda POKEYCHECKLO,x
         sta pokeybaselo
         lda POKEYCHECKHI,x
         sta pokeybasehi
         jsr checkforpokey
         lda pokeydetected
         beq foundpokeychip
         dex
         bpl detectpokeyloop
foundpokeychip
         sta pokeydetected
         rts

POKEYCHECKLO
        .byte <$4000, <$0450
POKEYCHECKHI
        .byte >$4000, >$0450

checkforpokey
         ldy #$0f
         lda #$00
         sta pokeydetected
resetpokeyregistersloop
         sta (pokeybase),y
         dey
         bpl resetpokeyregistersloop

         ldy #PAUDCTL
         sta (pokeybase),y
         ldy #PSKCTL
         sta (pokeybase),y

         ; let the dust settle...
         nop
         nop
         nop

         ; we're in reset, so the RANDOM register should read $ff...
         ldy #PRANDOM
         lda (pokeybase),y
         cmp #$ff
         bne nopokeydetected

         ; take pokey out of reset...
         ldy #PSKCTL
         lda #3
         sta (pokeybase),y
         ldy #PAUDCTL
         lda #0
         sta (pokeybase),y

         ; let the dust settle again...
         nop
         nop
         nop

         ; we're out of reset, so RANDOM should read non-$ff...
         ldy #PRANDOM
         lda (pokeybase),y
         cmp #$ff
         beq checkpokeyoncemore ;one $ff might be a fluke. try again...
         rts

checkpokeyoncemore
         lda (pokeybase),y
         cmp #$ff
         beq nopokeydetected
         rts

nopokeydetected
         dec pokeydetected
         rts