Spectranet: Tutorial 6

The Spectranet ROM provides some facilities for extending the ZX BASIC interpreter.

At present, there is an interpreter extension mechanism, and there are plans to add support for channels and streams at a later date. This tutorial will introduce BASIC extension methods.

How BASIC extensions work

When the ZX BASIC interpreter encounters something it considers an error, it executes the RST 8 instruction with the error code as the byte following the RST 8 instruction. The CPU calls the address at 0x0008, where the error routine examines the stack, and picks off the byte at the return address (the error code). It then processes it in the appropriate way.

The Spectranet traps execution at this address (unless this has been disabled by a hardware jumper or in software). This means the Spectranet ROM code runs at this point, and this gives an opportunity to look at some alternate syntax for new commands. If there's no command to run, control gets passed back to the ZX ROM to handle the error as it would have, had the trap not been present. The Spectranet ROM examines a table of commands, and if the command that the ZX ROM threw out matches one in the Spectranet's table, it calls a routine for the new command. For simple commands, this routine will simply execute the action intended. For more complex commands, there may be further parsing to be done.

When the new command is being executed, the Spectranet ROM is paged in. However, there's a simple mechanism to make calls to the BASIC ROM which will automatically page the Spectranet RON back in on return. This has been modeled on the way the ZX Interface 1 dispatches calls to the ZX BASIC ROM, to allow commands written to work with the Interface 1 to run on the Spectranet. It also means that the examples in the Complete Shadow ROM disassembly will work with very few modifications.

New commands can be written either in assembly language or C (or a mixture of C and assembly language). Commands can run from within the lower 16K of address space (for example, as part of a Spectranet ROM module or from static RAM), or in the Spectrum's main RAM. Code that's to run from Spectranet memory should be assembled into the area between 0x2000 and 0x2FFF (paging area B).

Adding a simple extension

(todo)

Further reading

(todo)