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⌂ ∞ ▽ Elite on the 6502

Loader: CopyZeroPage

[Commodore 64 version, Disk Loader 2]

       Name: CopyZeroPage                                            [Show more]
       Type: Subroutine
   Category: Loader
    Summary: Copy a page of data in a specified direction between zero page and
             the page at $CE00, omitting the first two bytes

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Elite GMA loader (Part 4 of 4) calls CopyZeroPage




 Arguments:

   C flag               Configures the source page:

                          * Clear = copy data from zero page to $CE00

                          * Set = copy data from the page at $CE00 to zero page



.CopyZeroPage

 LDX #2                 ; We copy the contents of either $0002 to $00FF (if the
                        ; C flag is clear) or $CE02 to $CEFF (if the C flag is
                        ; set), so set an index in X to start from offset 2

.swap1

 LDA $0000,X            ; Set A to the X-th byte of zero page

 BCC swap2              ; If the C flag is clear then skip the following
                        ; instruction

 LDA $CE00,X            ; Set A to the X-th byte of the page at $CE00

.swap2

 STA $0000,X            ; Store the copied byte in the X-th byte of both zero
 STA $CE00,X            ; page and the page at $CE00

 INX                    ; Increment the byte counter

 BNE swap1              ; Loop back until we have copied all of zero page

 RTS                    ; Return from the subroutine