Recently I found z88dk, “the C and assembler development kit that comes ready out-of-the-box to create programs for over 100 z80-family (8080, 8085, gbz80, z80, z180, ez80_z80, KC160, Rabbit 2000, 3000, 4000, 5000) machines”. Given that z88dk can create binaries for AgonLight computer which I have now (albeit only in Z80 mode with 64k of memory accessible), I naturally got interested.

First attempt

Let us take the following assembler file zfirst.asm:

CHPUT equ 00A2h

        ld      a,'A'
        call    CHPUT

Compiling it with zcc +msx -subtype=rom2 -o zfirst.rom zfirst.asm yields an error about undefined symbol _main. Of course, z88dk links the executable with crt0 which expects (as it would happen with C program) that the entry point _main is defined.

This is the next installment in my MSX Assembly 101 series. We are going to develop here printd routine, which will print the decimal value of the byte is the accumulator.

Comparing to printh (hexadecimal representation) there is an additional complication: hexadecimal representation always contains two digits (for example, the byte 0x01 is printed as 01, with leading zero); the number of digits in the decimal representation depends on the value of the number:

This is another post in the series about Z80 assembly (on MSX).

Previous posts:

1. Introduction
2. Printz
3. Using YAZE
4. Stack

In this post I will describe a creation of routine printh which will print hexadecimal representation of a byte in the accumulator register a.

Setup

I am going to make the code which can be compiled both as a cartridge and as a CP/M com file. The setup for this is described in the post Using YAZE and expaned in post Stack. Moreover, I am going to write the code in two parts: printh implementation itself and some code to “test” it by running.

Stack in cartridges

According to MSX Technical Handbook, a cartridge software should initialize stack pointer before using the stack. The common practice is to put 0xF380 into sp register (see this) and remember that stack grows downwards.

It means that any cartridge program, which uses the stack, should near its beginning do something like ld sp, 0f380h.

Stack in CP/M programs

When a CP/M program starts execution, sp points to the stack of 16 bytes. Since any subroutine call puts a return address on the stack, you can do no more than 7 nested calls if you do not use stack and do not call BDOS routines (2 bytes are already consumed by the CP/M return address, used when your program does final ret). Due to this, it is a good practice to use own stack space in CP/M program. Moreover, CP/M expects that the program preserves stack pointer. It all means that the following should be done at the start of a program:

Recently I found myself spending days (it will be one month with two weeks break in the middle) in a place, where I have an internet connection but not a computer. I do have an iPad with a keyboard, though, and it has SSH- and VNC-clients. Unfortunately, there are too many hops to home and too many hoops to jump (including port-forwarding over reverse SSH tunnel) to use VNC comfortably. This means that I cannot use MSX emulators. However, I wanted to continue my experiments with Z80 programming – over SSH.

This post will continue my MSX Assembly 101 series, started here.

We are going to develop the routine printz for displaying to the screen 0-terminated strings.

Control flow

Z80 CPU supports two types of jumps: jr and jp. There are also two commands for working with subroutines: call to call a subroutine and ret to return from it. Each of these command can take a condition, making control flow transition conditional on some flag. For example, ret z will return from the subroutine, but only if the flag Z is set.

This is hopefully the first post in a series devoted to assembly programming for MSX computers.

A little bit of history

When I was a schoolboy (and now I am over 50) we had Yamaha computers in school. They were implementing MSX standard and had Zilog Z80 CPU. Recently I have got interested in retrocomputing, and, while I do not have older hardware, there are multiple emulators, and I can even run MSX emulator on my Android TV (which has Bluetooth keyboard connected to it). Playing with MSX I feel like I am young and back at school again.