An 8-Bit Computer

Introduction

Before reading the MOS it helps to know what it is driving. An 8-bit home computer is a handful of chips on one board, and nearly everything interesting about the MOS comes from how those chips are wired together: what sits at which address, which chip owns which port, and what happens when two things want the same address.

The processor

At the centre is a Z80A. You know it from the assembly course: eight 8-bit registers that pair into 16-bit ones, a stack, and an instruction set that can read and write memory and talk to ports.

Two numbers define what the Z80 can reach:

Memory addresses are 16 bits. There are 65,536 of them, 0000h to FFFFh - the famous 64K. Every byte of program, data and screen the Z80 touches with LD has one of these addresses.
I/O ports are a separate set of addresses, reached only by IN and OUT. Writing to port 9 and writing to memory address 9 are different things that happen to share a number.

Memory: 64K of RAM, and 8K of ROM

The Einstein has RAM across the whole 64K. Your programs run in it from 0100h; the MOS keeps its working storage near the top, from FB00h.

It also has the ROM: 8,192 bytes - 8K - that hold the MOS. A ROM keeps its contents with the power off, which is why it exists at all: when you switch on, RAM is empty, and the processor needs some program already in place to run.

That raises a question an 8-bit designer cannot avoid. If RAM fills all 64K of addresses, where does the ROM go? The Einstein's answer is to put it at the same addresses as the bottom of RAM and switch between them: port 24h flips the bottom 32K, 0000h-7FFFh, between ROM and RAM. With the ROM switched in, addresses 0000h-1FFFh read the MOS. With RAM switched in, they read whatever your program put there. S4 is about what that costs and how the MOS lives with it.

Ports: one chip each

Everything that is not memory is a chip with a range of ports:

Ports	Chip	What it is for
00h-07h	AY-3-8910 sound generator	sound, and - unusually - reading the keyboard
08h-0Fh	TMS9129 video display processor (VDP)	everything on screen
10h-17h	8251 USART	the serial port
18h-1Fh	WD1770 floppy disc controller	the disc drives
20h-25h	single control ports	interrupt masks, the alpha-lock light, drive select, and 24h, the ROM/RAM switch
28h-2Bh	Z80 CTC (counter/timer)	the serial baud rate, and a clock that ticks once a second
30h-33h	Z80 PIO	the printer and the user port
38h	ADC0844 analogue-to-digital converter	the analogue inputs

The VDP is a good example of the pattern. It has its own 16K of memory for the screen, which the Z80 cannot address at all; the only way in is through ports 8 and 9. Every character the MOS prints ends up as bytes pushed out of port 8.

The code

You can talk to a chip directly, without the MOS, with two OUTs. This writes the VDP's register 7, whose low four bits are the colour behind everything on screen:

        ORG 256

        LD A,08h        ; the value: colour 8, medium red
        OUT (9),A
        LD A,87h        ; 80h + the register number, 7
        OUT (9),A
        RET

A VDP register is written as two bytes to port 9: the value first, then 80h plus the register's number.

Build and run

Press Run, as in the assembly course.

What you should see

The whole screen turns medium red behind the text, and the register display and > appear as usual - your program returned to the MOS.

The Einstein's screen with a red backdrop

Nothing in the MOS was involved in the colour change. Your program spoke to the chip, and the chip did it. Most of the MOS is exactly this, wrapped up so that a program does not need to know the ports.

Summary

The Einstein is a Z80 with RAM across its 64K, an 8K ROM that shares the bottom of the address space with RAM, and a chip at each range of ports. A program can drive any chip directly; the MOS exists so that it does not have to.

S3 asks the question this section left hanging: when you switch on, the RAM is empty - so what does the Z80 run first, and what must that program do?

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