Sunday, 10 November 2013

EPROM images for switchable Timex Sinclair 2068 modification

These are the images for the EPROMs used in the switchable TS2068 modification.

Switchable TS2068 27256.rom

Switchable TS2068 27128.rom

The Timex halves of the two EPROMs are completely standard TS2068 ROM images. The Spectrum half of the HOME EPROM is a very slightly modified version of the Spectrum ROM, modified to reset port 0xF4 and 0xFF on startup. The Spectrum half of the EX EPROM is a very slightly modified version of the Timex EX ROM, modified to jump to the START-NEW location in the Spectrum ROM on startup instead of the Timex HOME ROM location.

These modifications are necessary to ensure that the computer starts up every time reliably when in Spectrum mode. Unfortunately it is not possible to boot reliably on a TS2068 from a 100% exact Spectrum ROM.

In the Spectrum 48K half of the 27256 EPROM image, the standard Spectrum 48K ROM is patched as follows:

11D0    3E 3F    LD A, 0x3F
11D2    ED 47    LD I, A
11D4    00    NOP
11D5    00    NOP
11D6    00    NOP
11D7    00    NOP
11D8    00    NOP

becomes:

11D0    AF    XOR A
11D1    D3 F4    OUT (0xF4), A
11D3    D3 FF    OUT (0xFF), A
11D5    3E 3F    LD A, 0x3F
11D7    ED 47    LD I, A

In the Spectrum 48K half of the 27128 EPROM image, the standard TS2068 EX ROM is patched as follows:

0057    C3 31 0D    JP 0x0D31

becomes:

0057    C3 CB 11    JP 0x11CB

2 comments:

Chapas said...

"Was that the olny changes?"

I have disassembled your "Switchable TS2068 27256.rom" and the "TC2048.rom" and that was the only difference like you posted; but not with TS2068 OR TC2068, there are far more diferences.

Can you please clarify? Trying to do this mod on my TC2068.

Regards,
Chapas

Chapas said...

Spectrum it's from $0000h - $3fffh
Timex 2068 from $4000h - $7fffh

Is this correct?

Can you provide info how you build your ROM, disassembler, assembler, etc?

Thanks