Tarmac

Background

Tarmac is an incomplete dynamically recompiling emulator of an ARM microprocessor. It aims to emulate ARM architecture version 2a (an ARM3 processor) by decomposing sequences of ARM instructions to many sub-instructions known as armlets. These armlets are then more easily recompiled to x86 code with specialised optimisations being applied throughout the conversion. The x86 code is then executed to emulate the ARM instructions. By caching the generated x86 code and reusing it when the same sequence of ARM instructions is executed again, a significant speed up of the emulation can be achieved. The project was always intended to be used in the emulation of a full computer system meaning that real world problems such as synchronisation with subsystems, interrupts and exceptions have all been taken into account.


Red Squirrel

Part of the project was to develop an interpreting ARM emulator which is accurate enough to emulate the RISC OS Operating System. This was done in conjunction with Graeme Barnes' Red Squirrel, an Acorn computer emulator for Windows which provides the Memory map, IO and video emulation needed to emulate complex programs.

Screenshots of Tarmac's interpreting ARM emulator and Red Squirrel running RISC OS


BeebEm

Jon Welch borrowed the interpreting emulator and disassembler from Tarmac for use in BeebEm emulating the Acorn ARM Second processor board of a BBC Micro. Jon's made the original ARM Evaluation disk images and manual available to download which is well worth getting if you intend playing with this.

Strictly speaking, Tarmac doesn't emulate the ARM1 processor used in that board but the ARM3 emulation implemented is almost identical (only adding the MUL and SWP instructions). The second screenshot that follows shows the ARM BASIC assembler failing to recognise the MUL instruction.

Screenshots of Tarmac's interpreting ARM emulator and BeebEm running ARM BASIC on a BBC Micro

To use the ARM emulation on BeebEm:

Select menu item: Hardware / BBC Model / BBC Master 128
Select menu item: Hardware / ARM Second Processor

Now you're ready to use the machine:
Type 'CON. TUBE' to use the ARM co-processor by default
Type 'CON. FILE 13' to make ADFS the default filing system (so you can read the disc image).
Type 'CON. MODE 0' to switch to a screen mode with smaller font.
Press Ctrl + F12 to reset the machine so configuration changes take effect (presses Ctrl+Break in the emulator).

To make sure all your settings are saved for next time so you don't have to repeat the process:
Select menu item: Options / Save Preferences

Select menu item: File / Load Disc 0
Then browse and select armdisc3.adl supplied.

Now you're ready to run some software:
Type 'MOUNT 4' to mount disc.
Type '*.' to list files on disc.
Type 'AB' to start ARM BASIC
Type 'HELP .' to list all BASIC commands


Current Status

The interpreting emulator and disassembler are both usable in their current state though may contain a couple of bugs.

The actual dynamic recompiler was not completed in the time available for the project, though nearly all the framework is in place. Code is generated for many ARM instructions and several optimisations are performed. A comprehensive report of the background to the project with an explanation of the techniques used and examples of recompilation, as well as full C++ source code is available. This software was developed for my final year project as part of my degree in Computer Science at the University of Warwick.


Download

Report 3.35Mb PDF File
Development Diary 82 Kb PDF File
Latest source code
Source code May 2001 73 Kb Zip Archive


Links

RPCemu

In 2007 Tom Walker added a dynamically recompiling StrongARM emulator to his open source RPCemu which emulates an Acorn RISC PC. In the same way as Red Squirrel, Tom took a more direct approach, generating x86 or x64 machine code directly and getting very good performance as a result. Full source code is available from the subversion repository.

Armphetamine

The year before Tarmac was developed, Julian Brown wrote a dynamically recompiling ARM emulator called ARMphetamine. Although more complete than Tarmac since it recompiles all ARM instructions to x86 code which does actually execute, at the time of writing it too is buggy and incomplete in terms of support for the nastier side of emulating a real machine.

Red Squirrel

Following the collaboration on Tarmac, Graeme Barnes went on to develop his own JIT recompiler from ARM to x86 code for Red Squirrel. It takes a simpler approach, not performing many optimisations, but in doing so is far more successful and can run real world code for various versions of ARM processor.