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⌂ ∞ Elite on the BBC Micro

Utility routines: SWAPZP

[BBC Master version]

       Name: SWAPZP                                                  [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Swap zero page (&0090 to &00EF) with the buffer at &3000

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BRBR calls SWAPZP
             * CATS calls SWAPZP
             * DELT calls SWAPZP
             * GTDIR calls SWAPZP
             * LOAD calls SWAPZP
             * SAVE calls SWAPZP




.SWAPZP

IF _COMPACT

 JSR NMICLAIM           \ Claim the NMI workspace (&00A0 to &00A7)

ENDIF

 LDA #%00001111         \ Set bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch screen memory into &3000-&7FFF

 LDX #&90               \ We want to swap zero page from &0090 and up, so set an
                        \ index in X, starting from &90

.SWPL1

 LDA ZP,X               \ Swap the X-th byte of ZP with the X-th byte of &3000
 LDY &3000,X
 STY ZP,X
 STA &3000,X

 INX                    \ Increment the loop counter

 CPX #&F0               \ Loop back until we have swapped up to location &00EF
 BNE SWPL1

 LDA #%00001001         \ Clear bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch main memory back into &3000-&7FFF

 LDA #6                 \ Set bits 0-3 of the ROM Select latch at SHEILA &30 to
 STA VIA+&30            \ 6, to switch sideways ROM bank 6 into &8000-&BFFF in
                        \ main memory (we already confirmed that this bank
                        \ contains RAM rather than ROM in the loader)

 RTS                    \ Return from the subroutine