Why MacPaint's Original Canvas was 416 Pixels Wide

Re:Why MacPaint's Original Canvas was 416 Pixels Wide by klodolph on Monday December 30, 2013 @03:29AM (#6983693)

> In addition, there's no reason why one MOVEM can't contain data from multiple rows.

The screen buffer is 512x384 (24.5 KB, not 10.4 KB), so you have to copy one row at a time. The source and destination pointers have to increment by different amounts after each row (13 and 16 words, respectively). See the following lines in the article's assembly:

    ADD #52, A0
    ADD screenRow, A1

screenRow must be defined to 64.

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Re:Why MacPaint's Original Canvas was 416 Pixels Wide by teamonkey on Monday December 30, 2013 @10:37AM (#6984988)

A fair point, although I think the M68K was fast enough to copy the remaining pixels with a few MOVE.L calls if need be.

[Edit] by which I mean, yes, you're right that the MOVEM can't cross line boundaries in this case, but the CPU should be fast enough to deal with longer lines by using a MOVEM and adding a few extra MOVEs to copy individual longwords. Shorter lines would simply require using fewer registers in the MOVEM.

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