Indexed zeropage addressing
Indexed zeropage addressing is similar to Indexed absolute addressing, except that the base address given is in zero-page.
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In Indexed zero page addressing the contents of either the X or Y index register are added to a given, zero-page base address, to obtain the "target" address. This is useful in loops in which a number (256 or less) of bytes in memory is to be given similar "treatment". Here is an example in assembler, which uses a loop with the X index register to copy 50 bytes starting at $A0 in zero-page into 50 bytes beginning at label Target:
LDX #49 ;Use X index register as "counter" Loop: LDA $A0,X ;Get a byte from Source + X STA Target,X ;Store it at Target + X DEX ;Count down one byte BPL Loop ;If not negative (-1, FFh), we're not done, repeat from label Loop
In this example, the LDA instruction is using Indexed zeropage addressing mode: The LDA fetches a byte from an address that is calculated as the address of $A0 plus the contents of the X index register. The STA instruction is using the Indexed absolute addressing mode, storing the value in the Acculumator at the address of label Target plus the contents of the X index register. Since the X register acts as a counter in the loop, counting up from 49 thru 0, this reading and writing of bytes takes place on 50 consecutive bytes in memory. When X reaches -1 the BPL at the end no longer takes the branch back to label Loop, and the system "escapes" the loop.
Since an index register can only hold single-byte, unsigned integers in the range from 0 thru 255, the above routine can handle no more than 256 bytes. To process more bytes in a loop structure like this requires the use of indirect-indexed addressing.