Feeding the Genie

The einSDein interface is quite versatile. At its connector it is simply a very basic set of Z80 bus and control lines.

RD, WR,IORQ, M1. A[0..7] and D[0..7].

So it shouldn't be hard to interface to other Z80-based computers, right?

Right!



Here is an issue 2 einSDein wired up to a Video Genie, the UK version of the popular TRS-80 clone made by the EACA corporation. It is variously known by other names such as the Dick Smith System 80, PMC-80, TRZ-80 and so on.

The expansion connector breakout was custom made one rainy afternoon, and while it looks a mess it does the business.

A small machine language program was written to perform a directory listing of the SD card. This was converted into a cassette image format, then loaded using PlayCAS.

Here is a video on arguably the best known video distribution channel in the western hemisphere. It shows the aforementioned program pulling a listing from the interface pictured above.

The more eagle-eyed may recognise that the programs on the card are not Genie programs. This is being attended to, dear reader. I have written a converter program which takes cassette image format files and spits out a raw-ish binary dump which can be loaded from SD card. It only works with 'system' or machine language programs at the moment, but a gander at the Level II ROM reference book will soon see to that restriction.

Armed with a copy of MAME and its fine debugger I will spend a few quality hours stepping through the cassette loading functions to see what I need to do in order to craft a work-alike in order to load BASIC from the SD card.

He's at it again!

I'm getting back into the Atom swing! After a fruitless hour measuring incorrect logic levels at the Atom's expansion port I was on the verge of giving up. I had been prodding the VIA, according to available documentation, and getting nowhere. All I wanted was a toggly bit. Was that too much to ask for? That's when I decided to do some chip swapping.

It takes me longer than most people to get to this point as I'm a software person at heart and so blame that first ;)

I just happened to have a vintage 6522 VIA lying on my desk so I swapped the chips over. Joy! I was flipping bits like a demon. Naturally my thoughts turned to MMC...

10@=0
20!#208=!#208+3
30V=#B800
40W=#FFF4
50T=#80
60DIM LL(13),C(6)
70F.N=0TO13;LL(N)=#FFFF;N.N
80F.N=1TO2; DIM P(-1)
90P."PASS "N',$21
100[
110\ main
120:LL0 LDA @#0
130STA C+1; STA C+2; STA C+3; STA C+4
140LDA @#95; STA C+0
150JSR LL3
160LDA @#40+#0; JSR LL7
170LDA @CH"1"; SBC @0; JMP W
180\ select card
190:LL1 LDA @0; STA V; RTS
200\ deselect card
210:LL2 LDA @#20; STA V; RTS
220\ init hw and go spi
230:LL3 LDA @0; STA V+#C
240LDA @#FE; STA V+2
250LDA @#20; STA V
260LDY @10
270:LL4 LDA @#FF; JSR LL5; DEY; BNE LL4
280RTS
290\ xferbyte
300:LL5 STX T+0; STY T+1; LDY @8
310:LL6 PHA; AND @#80; STA V
320ORA @#40; STA V
330LDX V; AND @#BF; STA V
340TXA; ROR A
350PLA; ROL A
360DEY; BNE LL6
370PHA; JSR #F7FA; PLA
380LDY T+1; LDX T+0;
390RTS
400\ command
410:LL7 STA C+5; LDY @5
420:LL8 LDA C,Y; JSR LL5; DEY; BPL LL8
430:LL9 LDA @#FF; JSR LL5; AND @#FF; CLC; BPL LL10
440DEY; BNE LL9;
450SEC
460:LL10 RTS
470]
480P.$6
490NE.N
500END

If you're an ex-atommer or you've been around BBC micros you might recognise this. If you don't then I can tell you it's some Atom basic with inline assembler. The Atom's Basic dialect was hard on the eyes but lightning fast. By the standards of the day, naturally ;)

The built-in assembler gave the user of this machine an instant boost when learning to program. No fussing with 3rd party apps, loading the assembler or machine code, or the attendant problems with cassettes. Turn on. Assemble. Bam! (Which is as it happens almost quite literally what happened to my 1st Atom over 20 years ago...) Now if only it was that easy for the kids of today - it was much better in the old days - I remember when all this was fields etc. etc.

The above listing is code to bit-bang SPI to an MMC card attached via the simplest of hardware harnesses to the rear expansion connector of the Atom. I'm looking at making a ROM based solution that hooks into the OS's filing system vectors. It's all quite primitive at the moment but I'm hoping it'll solidify. Get in touch if you have any experience of developing this kind of Atom program. I'd love to talk to you.