collapseos/TRICKS.txt

This file describe tricks and conventions that are used throughout the code and
might need explanation.

*** Quickies

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 flag for results

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.

*** Little endian

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.

*** DAA

When it comes to dealing with decimals, the DAA instruction, which look a bit
obscur, can be very useful. It transforms the result of a previous arithmetic
operation involving two BCD (binary coded decimal, one digit in high nibble,
the other digit in low nibble. For example, 0x99 represents 99) into a valid
BCD. For example, 0x12+0x19=0x2b, but after calling DAA, it will be 0x31.

To clear misunderstanding: this does **not** transform an arbitrary value into
BCD. For example, "ld a, 0xff \ daa" isn't going to magically give you a binary
coded 255 (how could it?). This is designed to be ran after an arithmetic
operation.

A common trick to transform an arbitrary number to BCD is to loop 8 times over
your bitstream, SLA your bits out of your binary value and then run
"adc a, a \ daa" over it (with provisions for carries if you expect numbers
over 99).