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Grant Searle's (not-so-) Simple-Z80 :-)

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Grant Searle's "Simple Z80" in its 32k format is a great computer, reminding me very much of my old Nascom-1 back in 1978. Its true 'Retro' and quick, easy and cheap to make using not-too-hard-to-find parts, and Jeff Tranter has kindly made his PCB available for it via easyeda.

As designed, its a BASIC-only machine which is brilliant for playing Star Trek and Sargon Chess, but I always like to have raw access with a machine-code monitor - such as the T2 in my old Nascom and NAS-SYS in later models.

The 8k BASIC has had a few redundant routines removed, such as cassette handling, keyboard and screen display, so its actually slightly smaller than its original 8k. In fact within the ROM, the interpreter finishes at address 1DB7 hex and 1DB8 to 1FFF are unused - leaving 584 ROM bytes free!

Within the confines of the 8k EEPROM there would be room for a modest monitor if I kept it simple. On the other hand it had to have all the essential commands to be of any genuine benefit - in fact its all worked out very nicely although it is an 'only just' fit!

The update is in three parts, first within the interrupt I/O driver the "Warm or Cold?" signon message has been extended to "Warm, Cold or Monitor?" and in the BASIC code, the command 'monitor' has been enabled.  For the monitor itself I tried a different cross-assembler this time, AZ80, and I quite like it as it has no syntax peculiarities unlike some :-)
With the addition of an 'era typical' monitor, the 32k “Simple Z80” becomes a really handy little hobby computer with much more of a “Nascommy” feel, and it presents the opportunity to try a few machine-code routines, hand-coded or cross-assembled elsewhere. I've had great fun with mine, thank you Grant!

A new rom image suitable for the 27C64 EPROM or as in my case an AT28B64 EEPROM is posted on in the 'retro' section, along with a brief PDF document which details everything you see here.

Here's a slightly outdated video (its since been tidied a little):



Did you write the BASIC interpreter, or was borrowed from an existing one?

Hi, no its the Microsoft BASIC adapted for the Nascom in 1978, which Grant has tweaked to suit his board:
I just did the I/O changes and wrote the monitor  ;D
Details & a brief document are in the retro section of (These files were updated Nov 5th '21)

Should anyone be following this, please pull a fresh copy as I've added a page to the document regarding calling machine code routines and passing parameters between M/C & BASIC.  Also tidied up the document a bit and put some more links & credits in.

Since the video was taken, I've swapped the Arithmetic command around to make it more logical. On Nas-Sys it always seemed the wrong way round to me:
Axxxx yyyy on Nas-Sys gave you the sum xxxx + yyyy, the difference yyyy - xxxx (wrong way around to my mind!)
and the relative jump offset. Mine does xxxx+yyyy, xxxx-yyyy and offset. Makes more sense to me :-)


My board arrived and I managed to find most of the necessary parts in the 'junk' pile, the only part I'm missing is a nice fast Z80 but this ST part does seem to be doing something


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