Z80 Assembly programming for the ZX81!

The ZX81 was one of the earliest affordable home computers, and was actually the first computer my family owned!

While my only memories of it was it crashing while loading hangman, we can have some fun learning about this early Z80 computer!


ZX81
Cpu Z80 3.25mhz
Ram 1k (!) 32k upgradable
Vram ha! (768 bytes)
Resolution 32x24 characters
Charset 64 characters + Inverse
Colors Black and white
Sound None



The Keyboard Layout


Reading the keyboard

We can read in a row of the keyboard by setting BC to the correct value and using IN A,(C)... the resulting byte will contain a 0 if the key is pressed, 1 if it is not.


C= &FE
B=... 0 1 2 3 4
%11111110 SHIFT Z X C V
%11111101 A S D F G
%11111011 Q W E R T
%11110111 1 2 3 4 5
%11101111 0 9 8 7 6
%11011111 P O I U Y
%10111111 ENTR L K J H
%01111111 SPC DEL    M       N       B   

The Character Map

Below are the visible characters... Note that basic commands and other functions are also 'characters', the most important is &76 (118) which is NewLine - we need this for the 'Display File' (Screen Ram)


Ports

Port
Purpose
FB Printer Write
FB R Printer Read
FD W Disable NMI (used for FAST mode)
rrFE R Init Vertical Retrace / Casette out LOW / Read Keyboard row rr
FE W Enable NMI (used for SLOW mode)
FF W Terminate Vertical Retrace and Restart LineCTR / Cassette out HIGH

Memory Map

From To Purpose
0000   3FFF   ROM
4000 4008 System Vars
4009 407C P-File save area
407D 43FF 1K RAM
4400 FFFF 16K+ Ram upgrade

VRAM is at the end of your basic program, it's address is stored at &400C



 

View Options
Default Dark
Simple (Hide this menu)
Print Mode (white background)

Top Menu
***Main Menu***
Youtube channel
Patreon
Introduction to Assembly (Basics for absolute beginners)
Forum
AkuSprite Editor
ChibiTracker
Dec/Bin/Hex/Oct/Ascii Table

Alt Tech
Archive.org
Bitchute
Odysee
Rumble
DailyMotion
Please note: I wlll upload more content to these alt platforms based on the views they bring in

Z80 Content
***Z80 Tutorial List***
Learn Z80 Assembly (2021)
Learn Z80 Assembly (old)
Hello World
Simple Samples
Advanced Series
Multiplatform Series
Platform Specific Series
ChibiAkumas Series
Grime Z80
Z80 Downloads
Z80 Cheatsheet
Sources.7z
DevTools kit
Z80 Platforms
Amstrad CPC
Elan Enterprise
Gameboy & Gameboy Color
Master System & GameGear
MSX & MSX2
Sam Coupe
TI-83
ZX Spectrum
Spectrum NEXT
Camputers Lynx

6502 Content
***6502 Tutorial List***
Learn 6502 Assembly
Advanced Series
Platform Specific Series
Hello World Series
Simple Samples
Grime 6502
6502 Downloads
6502 Cheatsheet
Sources.7z
DevTools kit
6502 Platforms
Apple IIe
Atari 800 and 5200
Atari Lynx
BBC Micro
Commodore 64
Commodore PET
Commander x16
Super Nintendo (SNES)
Nintendo NES / Famicom
PC Engine (Turbografx-16)
Vic 20

68000 Content
***68000 Tutorial List***
Learn 68000 Assembly
Hello World Series
Platform Specific Series
Simple Samples
Grime 68000
68000 Downloads
68000 Cheatsheet
Sources.7z
DevTools kit
68000 Platforms
Amiga 500
Atari ST
Neo Geo
Sega Genesis / Mega Drive
Sinclair QL
X68000 (Sharp x68k)

8086 Content
Learn 8086 Assembly
Platform Specific Series
Hello World Series
Simple Samples
8086 Downloads
8086 Cheatsheet
Sources.7z
DevTools kit
8086 Platforms
Wonderswan
MsDos

ARM Content
Learn ARM Assembly
Learn ARM Thumb Assembly
Platform Specific Series
Hello World
Simple Samples
ARM Downloads
ARM Cheatsheet
Sources.7z
DevTools kit
ARM Platforms
Gameboy Advance
Nintendo DS
Risc Os

Risc-V Content
Learn Risc-V Assembly
Risc-V Downloads
Risc-V Cheatsheet
Sources.7z
DevTools kit

MIPS Content
Learn Risc-V Assembly
Platform Specific Series
Hello World
Simple Samples
MIPS Downloads
MIPS Cheatsheet
Sources.7z
DevTools kit
MIPS Platforms
Playstation
N64

PDP-11 Content
Learn PDP-11 Assembly
Platform Specific Series
Simple Samples
PDP-11 Downloads
PDP-11 Cheatsheet
Sources.7z
DevTools kit
PDP-11 Platforms
PDP-11
UKNC

TMS9900 Content
Learn TMS9900 Assembly
Platform Specific Series
Hello World
TMS9900 Downloads
TMS9900 Cheatsheet
Sources.7z
DevTools kit
TMS9900 Platforms
Ti 99

6809 Content
Learn 6809 Assembly
Learn 6309 Assembly
Platform Specific Series
Hello World Series
Simple Samples
6809 Downloads
6809/6309 Cheatsheet
Sources.7z
DevTools kit
6809 Platforms
Dragon 32/Tandy Coco
Fujitsu FM7
TRS-80 Coco 3
Vectrex

65816 Content
Learn 65816 Assembly
Hello World
Simple Samples
65816 Downloads
65816 Cheatsheet
Sources.7z
DevTools kit
65816 Platforms
SNES

eZ80 Content
Learn eZ80 Assembly
Platform Specific Series
eZ80 Downloads
eZ80 Cheatsheet
Sources.7z
DevTools kit
eZ80 Platforms
Ti84 PCE

IBM370 Content
Learn IBM370 Assembly
Simple Samples
IBM370 Downloads
IBM370 Cheatsheet
Sources.7z
DevTools kit

Super-H Content
Learn SH2 Assembly
Hello World Series
Simple Samples
SH2 Downloads
SH2 Cheatsheet
Sources.7z
DevTools kit
SH2 Platforms
32x
Saturn

PowerPC Content
Learn PowerPC Assembly
Hello World Series
Simple Samples
PowerPC Downloads
PowerPC Cheatsheet
Sources.7z
DevTools kit
PowerPC Platforms
Gamecube

Work in Progress
ChibiAndroids

Misc bits
Ruby programming









Buy my Assembly programming book
on Amazon in Print or Kindle!


Buy my Assembly programming book



Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!




































Buy my Assembly programming book
on Amazon in Print or Kindle!


Buy my Assembly programming book



Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!




































Buy my Assembly programming book
on Amazon in Print or Kindle!


Buy my Assembly programming book



Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!