Fill an array with the numbers from 1 to 10
A djnz loop that writes ten bytes into reserved room, stepping the pointer itself, with the result read in the memory panel.
Ten bytes of room are reserved in memory and a loop writes the numbers 1 to 10 into them, one byte
per pass. The answer is in the memory panel: type 9000 in its address box and the ten bytes are
there, 01 to 0A.
The bigger of two numbers ran a fixed handful of instructions. This is the first program that runs the same three instructions over and over, and the first one that writes into memory the assembler put nothing in.
You need to know: the "Loops and djnz" lecture and the "org, db, dw and ds" lecture. What is new
here is that the destination of an ld can be an address held in a pair, (hl) writes the byte
where hl points and the inc hl under it moves hl on to the next element.
.ds count reserves ten bytes and writes nothing into them, so before the run the ten bytes at
0x9000 read 00, which is what untouched memory reads in this editor. ld hl, numbers puts their
address in hl, and from there the loop only ever talks about (hl), which is C's *p.
The 1 that inc hl adds is the size of one element, and it is yours to get right: nothing in
(hl) knows how big the thing it wrote was. Write a second inc hl under the first one and the
numbers land two bytes apart, with an untouched 00 between each pair and the last five written
past the end of the room that was reserved for them.
djnz runs the loop exactly count times, so b starts at 10 and no adjustment is needed.
That is the one place the Z80 is easier than the M68K, whose dbra stops at -1 and has to be given
count-1. The other side of it is that the counter is always b, and always counts down, so the
number being written lives in a second register.
Step through the loop and hl climbs by 1 at every inc, from 9000 to 900A, while b walks
down to 00, which is the zero that ended it. a comes out at 0B, one past the last number it
wrote.
Try changing inc a to two inc a lines. The array fills with 1, 3, 5 and the rest of the odd
numbers up to 13, which is 19, because the counter that ends the loop and the number being written
are two different registers doing two different jobs.