Reverse a string in place
Two pointers that start at the ends of a string and swap bytes on their way towards each other, with the swaps counted, since the Z80 cannot compare two pairs.
The string at 0x9000 is turned back to front where it lies, with no second buffer to copy it into.
Two pointers start at the two ends and walk towards each other, swapping the bytes they point at.
Length of a string walked to the terminator to measure something. This one walks to the terminator to find the far end and then does its work on the way back, so there are two pointers moving at once, and the length it measured on the way is what says when to stop.
You need to know: the "Length of a string" Example and the "Addressing on the Z80" lecture. What
is new here is a second pointer in de, which can be written through with ld (de), a and nothing
else, so the byte being moved has to pass through the accumulator.
The M68K writes this loop as "while left is below right", one cmp.l a0, a1 per pass. The Z80
has no comparison on a pair at all: cp compares against a and a is eight bits, and the only 16
bit subtraction, sbc hl, de, writes over hl, which here is one of the two pointers. So the loop
counts instead. The length is measured once, srl a halves it, and djnz runs exactly that many
swaps.
Halving with a shift is where the odd lengths are taken care of: srl a throws the bottom bit away,
so a string of five characters gives two swaps and its middle character is never touched, which is
what you want.
The swap needs both bytes in registers before either is written, which is why c holds one of them
while a carries the other. ld a, (de) and ld (de), a are the only two instructions that reach
memory through de, and neither of them will name any register but a, so the byte from the right
hand end goes into a first and then into c to get out of the way.
Run it with the memory panel on 9000 and the eight bytes read 79 6C 62 6D 65 73 73 41. Press the
text button in the panel's corner and they read ylbmessA.
Try changing the string to .asciz "Level": five characters, two swaps, the v in the middle stays
where it is, and the memory panel reads leveL.