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Add TRICKS.txt

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Virgil Dupras 2019-10-30 21:00:10 -04:00
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This file describe tricks that are used throughout the code and might need
explanation.
or a: Equivalent to "cp 0", but results in a shorter opcode.
xor a: sets A to 0 more efficiently than ld a, 0
and 0xbf: Given a letter in the a-z range, changes it to its uppercase value
if it's already uppercased, then it stays that way.
Z if almost always used as a success indicator for routines. Set for success,
Reset for failure. "xor a" (destroys A) and "cp a" (preserves A) are used to
ensure Z is set. To ensure that it is reset, it's a bit more complicated and
"unsetZ" routine exists for that, although that in certain circumstances,
"inc a \ dec a" or "or a" can work.