ZX Computings - OpenLibra

Machine code
First steps in Machine
code
There seems to be a myth surrounding
ZX81 machine code.
For some strange reason it is a
common belief that programming
in machine code is not easy.
Indeed there are even people
who would go as far as to call it
difficult. Suffice to say this is
not the case for, while it is true
that the use of machine code
was omitted from the Sinclair
Manual, all you need is one little
(BASIC) program taken from
the book. Mastering Machine
Code On Your ZX81. to get you
started. Here it is . . .
10
20
30
40
50
60
70
80
INPUT X
LET A*-" "
IF A$ » " " THEN INPUT A$
IF A* -"S" THEN STOP
POKE X. 16*CODE A$ +
CODE A»(2)-476
LET X - X + 1
LET AS AS(3 TO) note
there is nothing between
"TO" and "I"
GOTO 30
Now obviously there isn't
enough room in this article for
Machine code may seem
bewildering, but Toni Baker —
author of Mastering Machine Code
on Your ZX81 or ZX80 — is
convinced the whole subject is a
piece of cake. Here, Toni
attempts to convince you, with a
clever screen-handling routine
and the raw ingredients of a
BREAKOUT game.
me to teach the whole of
machine code from scratch —
that would be like trying to
squash Sinclair's BASIC
Manual into two or three pages
— so instead I'll give you one or
two little machine code
routines which you can use in
your own BASIC programs.
Try this one. Load the above
program, then add the following
line:
1 REM 123456789023456789
Now RUN the program and input:
16514
"2A0C40"
"0618"
"23"
"7E"
"EE80"
"FEF6"
"2803"
"77"
"18F5"
"10F3"
"C9"
Machine
code address
LD HL.
(D FILE)
LD B,
twentyfour
LOOP: INC HL
LD A.IHL)
XOR80h
CP F6h
HR Z.EXIT
LD (HL).A
JR LOOP
EXIT: DJNZ LOOP
RET
Now please note — you do not
need to understand what I've
written in the right-hand col
umn. All you need to do is enter
what's written in the left column.
When you've done that
type "S" to stop the program.
Now add these lines:
100 IF INKEY$ < >*' " THEN
GOTO 100
105 IF INKEYt = " " THEN GOTO
105
110 RAND USR 16514
120 GOTO 100
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