45 lines
1.5 KiB
Plaintext
45 lines
1.5 KiB
Plaintext
Copy screen + colour RAM to the right one place
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This sequence of instructions:
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loop:
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DEY
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LDA ($D1),Y
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INY
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STA ($D1),Y
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DEY
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LDA ($F3),Y
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INY
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STA ($F3),Y
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DEY
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CPY $D3
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BNE loop
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LDA #$20
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STA ($D1),Y
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is the obvious any minimal way to copy the screen and colour RAM data.
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The constants $D1 and $F3 are fixed by the publicly documented interface
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of the KERNAL to point to the screen and colour RAM of the screen.
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The Y register is the only register that can be used to post-increment an
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indirect address on the 6502.
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Thus the only efficient manner to copy a string to the right one byte
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is to read, increment Y by one to get the destination address, store,
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then decrement Y again so that it points to the source offset, now read
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the colour RAM byte, add one to Y so that we point to the destination,
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and then write it.
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Similarly the check against $D3 (current column of cursor on screen) is also
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necessary for working out when to terminate the loop. The storing of $20
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into the screen RAM is also required to effect the insertion of a space.
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In fact, about the only variant possible would be to swap the $D1 and $F3
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constants so that the colour RAM is copied before the screen RAM in each
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loop, instead of after it.
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It is telling that this routine was the result of independent implementation
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of this routine, without looking at the KERNALs routine, in terms of supporting
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the argument that the routine is the obvious manner in which to do this.
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