Introduction
A GBA with a disc drive
Keys
Things that work
Things that don't
Credits

Introduction

PocketBeeb is a BBC 'B' emulator for the GBA.

The BBC was a popular 8-bit micro computer used in schools in the UK in the 1980s. It was probably most famous as the birth place of Elite.

Originally PocketBeeb was conceived as a stand-alone emulator to run just one game: Exile. I've added bits on until the current version supports a not-too-shabby number of games using the common single-sided disc format (SSD) and is also compatible with BeebEm UEF save states.

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A GBA with a disc drive

In order to load discs on the GBA, you have to append the files onto the emulator. A program is included that makes adding ssd and uef files a fairly painless experience.

The steps are:

1. Download PocketBeeb
2. Download some BBC disc images
3. Run injector program
4. Select the discs you want to add
5. Create the PocketBeeb+discs ROM
6. Flash this to your GBA cartridge

Once PocketBeeb is up and running on the GBA, you will see either the first save state loaded (if you added a save state in step 4 above) or the BBC's "command prompt".

• Press Start & Select to bring up the PocketBeeb menu.
• Highlight "Load Disc" and press A.
• All being well, you should see a list of the discs you added
• Choose one and press A

The BBC resets and the disc starts to load... assuming it is an auto booting disc.
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Keys

The GBA has just 10 different control inputs. The BBC has a keyboard with over 50 keys. PocketBeeb overcomes this hurdle using a virual keyboard and mappable controls.

Pressing Start + Select together brings up the emulation options menu. From here you can redfine keys, load games, change video preferences, etc.

When creating the ROM, double clicking a game will bring up a window where you can select the keys that each GBA input will correspond to. These are the default controls used when you load the game on the GBA - you can remap them later but each time you reload, the default keys are restored.

Some games require multiple key presses (e.g entering hiscore names or passwords) the virtual keyboard can be used for this. Menu->Keys-> Press A on the VK option to turn it ON. When you exit the menu, the virtual keyboard is shown over the BBC screen.

d-pad	changes the selected key
A	'press' the virtual key.
B	hide virtual keyboard
R/L	change virtual keyboard position

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Things that work ok...

• "Fat pixel", low resolution modes (as seen in Exile, Repton, Magic Mushrooms, etc etc)
• Sound - 3 channels plus noise generator
• Multiload games (Repton 3, Ravenskull, etc)
• Palette changes (Felix + Weevils, Magic Mushrooms, etc, etc)
• Teletext, but barely enough to see the controls on Acornsoft games.
• Timers, as used in every game ever, especially Volcano and Snapper

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Things that don't work and probably never will....

Accurate emulation is possible on giga hertz PCs, on a 16Mhz GBA things get a bit tougher. Don't expect to see these things any time soon:

• Hi-res graphics - eg. Meteors, Frak - are not drawn correctly. How can I fit a 320*256 screen resolution into 240*160? Answer: with great difficulty
• Teletext Mode - this is a major fudge, most of it is not drawn correctly
• Palette changes - some games use very accurate timing to change the palette and provide extra colours on screen. These appear very flickery as I don't emulate the video accurately enough e.g Last Ninja, Stryker's Run. A hack is in place to stop the flickering, but it is not ideal
• Clever games - Uridium, Firetrack, Revs, Elite, etc - that do exciting things to the video registers are too much for my timing hacks.
• Funky keyboard interrupt usage - Stryker's Run title screen
• Tape emulation

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Credits

Basic, Acorn DFSi, OS 1.2 roms © 1981 Acorn Computers Ltd. Used without permission... but then who would you ask?

PocektNES authors - Emulation of the BBC's 6502 uses a modified version of the PocketNES core. (do no NES games use ADC/SBC with the D flag set then?)

BBC specific emulation is taken from numerous sources, but thankyous must go to the authors of BeebEm for making an emulator with such easy to read source code.

ZXAdvance/Foon - Though I've not used anything really specific from either of these - ZXAdvance's sprite based screen notwithstanding - they have influenced my approach for 8-bit to GBA ideas enough to warrant credit.

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