The_Emulator_to_Emulate_the_BBC_Micro_on_the_Amiga.pdf

TO EMULATE THE BBC MICRO ON THE COMMODORE AMIGA

Commodore

THE EMULATOR DOCUMENTATION VERSION 1.2

COMMODORE BUSINESS MACHINES (UK) LTD.

Commodore Business Machines (UK) Ltd.

Commodore House, The Switchback,

Gardner Road, Maidenhead,

Berks SL6 7XA

Introduction

THE EMULATOR is a program which will give your Amiga the

appearance of a BBC Model B with BASIC 2 and DPS. THE

EMULATOR will run BBC BASIC with the most commonly used

VDU and *FX calls, and call 6502 subroutines.

It runs as a PROCESS on its own screen within the Amiga multi-

tasking operating system, which means you can run THE EMU-

LATOR at the same time as other Amiga programs such as

editors and Workbench accessories. Therefore, if you have an

Amiga 2000 with a Bridgeboard and 3 Mbytes of memory, you can

run BBC applications, PC applications, and native Amiga applica-

tions at the same time — with no data transfer problems between

the environments. You can even run several EMULATORS at

once.

The principal objective in creating THE EMULATOR is to run

existing educational software, written in BBC BASIC or in a mix-

ture of BBC BASIC and 6502 assembler. This has been achieved

without any degradation of the Amiga's native advantages, such

as split screens and multi-tasking.

THE EMULATOR strategy has been to create a "bridge" from the

BBC operating system to the Amiga's operating environment. This

means THE EMULATOR can run nearly all "legal" BBC software

which uses BASIC and the OS. However, THE EMULATOR

cannot run "illegal" software that dodges the OS and goes directly

to the BBC screen memory or to the hardware. In practice, this

means THE EMULATOR can run nearly all educational programs

written in BASIC or a mixture of BASIC and assembler, but not

programs such as arcade-style games.

Principal components of THE EMULATOR software are as fol-

lows:

Beebos: A "bridge" from the BBC operating system through to the

Amiga graphics libraries, maths libraries, etc. Nearly all BBC calls

are supported, including OSBYTE (*FX calls).

DPS: "Disk Furtling System" — a bridge from DPS through to

AmigaDOS.

BASIC: A new implementation of BBC BASIC, re-coded from

scratch in 68000 assembler. It is as compatible as possible with

BBC BASIC 2.

6502: This emulation of the 6502 chip — written in 68000 assem-

bler — enables programs to call assembler subroutines.

Other elements in the software are a "CLI environment" entered

by *GO, and a simple 6502 monitor, entered by 'MONITOR.

This documentation assumes basic familiarity with the Amiga

environment as explained in the Amiga User Guide; it also

assumes familiarity with the BBC computer. Material which needs

a more advanced understanding of the Amiga are flagged as

"Tech notes" — ignore these if you want to.

To get started double-click on THE EMULATOR Icon. If you are

running on a single drive system you may be prompted to reinsert

your Workbench to allow THE EMULATOR to load certain Amiga

system liBraries. You will then be in THE EMULATOR.

Note that THE EMULATOR expects to find BASIC in the same

drawer as itself. If THE EMULATOR can't find BASIC a CLI

window will appear with a '*' prompt.

To PAUSE THE EMULATOR, perform a mouse-click anywhere in

THE EMULATOR screen. This will also present you with an

Amiga "drag bar" at the top of the screen which can be used to

pull the screen up and down, or rearrange it relative to the

Workbench and any other screens you may have open. With THE

EMULATOR in pause mode, it is possible to run various acces-

sories from the Amiga Workbench and Extras disk, including

Palette and GraphicDump utilities. To get rid of the drag bar and

run THE EMULATOR, click again anywhere in THE EMULATOR

screen below the drag bar.

To EXIT THE EMULATOR, enter the special command:

'CLOSEDOWN

Tech Note: In order to run properly, THE EMULATOR needs to

have at least 300 kbytes free memory, and to be able to find

'mathstrans.library' in its LIBS: directory. If these conditions are

not met, it will fail to open. To gain maximum speed in 6502 emu-

lator, THE EMULATOR uses one or two elegant routines — this

means THE EMULATOR will run on a system with a 68010 or

68020 unless you disable instruction caching (68020), and run a

utility to catch MOVE SR instructions (68010 and 68020).

The keyboard

A good idea when first entering THE EMULATOR is to press

CAPS LOCK — this will allow you to talk to BASIC sensibly.

THE EMULATOR pays attention to your Keymap and Preferences

settings as determined by your Amiga Workbench. Your

Preferences determine such attributes as display positioning and

key repeat delays — these can be changed from within THE

EMULATOR by *FX 11 and 12 if necessary. The numeric keypad

works as you would expect.

Certain keys are "stolen" by THE EMULATOR to have special

effects. These are as follows:

Function keys — as BBC function keys; F10 used for FO

Cursor keys — as BBC cursor keys for copy key editing

Backspace — acts like BBC DELETE

Del — acts like copy key

Help —acts like BREAK

Note that some protection has been applied to the BREAK facility.

To BREAK it is necessary to hold down the left Amiga key (CBM

key on some ASOOs) then press the 'help 1 key. This performs a

"warm boot" like BBC BREAK; you can also perform a "cold boot"

by holding down the 'Ctrl' key and left Amiga then pressing the

help' key. Cold boot differs from warm boot in that it resets the

programmable function keys and resets the DFS — note that this

involves a few seconds pause for disk activity.

Graphics modes

THE EMULATOR supports the following graphics modes:

0 640x256 2 colours 80x32

1 320x256 4 colours 40x32

2 320x256 16 colours 20x32

3 text only 2 colours 80x25

4 320x256 2 colours 40x32

5 320x256 4 colours 20x32

6 text only 2 colours 40x25

7 Teletext display 40x25

In modes 2 and 5 the horizontal resolution is actually double the

original. The double width (20 column) characters are rendered

accurately.

The BBC's Teletext (Mode 7) display is software emulated by THE

EMULATOR. Mode 7 is the fastest mode on the BBC, but the

slowest mode on THE EMULATOR. This is because the Amiga

has to emulate character-based hardware with pixel-based soft-

ware.

The speed of THE EMULATOR Mode 7 should be fast enough for

all practical application purposes; however, it is not recommended

for program editing. For this reason, the "power up default" for

THE EMULATOR has been set to Mode 6.

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