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tricks: add endianness notes

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Virgil Dupras 2019-11-04 16:49:53 -05:00
parent 9344c4b961
commit 6396eb4a9e
1 changed files with 14 additions and 2 deletions
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TRICKS.txt
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@ -1,5 +1,5 @@
This file describe tricks that are used throughout the code and might need
explanation.
This file describe tricks and conventions that are used throughout the code and
might need explanation.
or a: Equivalent to "cp 0", but results in a shorter opcode.
@ -13,3 +13,15 @@ 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.
z80 is little endian in its 16-bit loading operations. For example, "ld hl, (0)"
will load the contents of memory address 0 in L and memory address 1 in H. This
little-endianess is followed by Collapse OS in most situations. When it's not,
it's specified in comments.
This get a bit awkward with regards to 32-bit. There are no "native" z80 32-bit
operations, so z80 doesn't mandate an alignment. In Collapse OS, 32-bit numbers
are stored as "big endian pair of little endian 16-bit numbers". For example,
if "ld dehl, (0)" existed and if the first 4 bytes of memory were 0x01, 0x02,
0x03 and 0x04, then DE (being the "high" word) would be 0x0201 and HL would be
0x0403.