Step50: Customizing Amiga 500

As I worked extensively on that great computer, I made some modifications to it. Some of these changes were very convenient. Here is a list of them.

Level 7 Interrupt

The Level 7 interrupt is not used in the A500. It is the highest level interrupt. You can generate this signal by soldering a switch onto the CPU. When you trigger it, you can freeze the Amiga, effectively allowing you to "pause" the game.

The assembler tool (AsmOne) had a function to catch the Level 7 IRQ. When triggered, the current code exits and returns to AsmOne. This was very handy, eliminating the need to code clean mouse tests and exits.

I do not have any photos of my setup. I believe it only requires connecting a switch to the IRQ 7 input of the 68000 chipset.

5 1/4 disks on Amiga

I had two 5 1/4 drives on my Amiga. The 5 1/4 floppies were less expensive than the 3 1/2 ones. I had nearly 1,000 to 2,000 floppies under my bed, mainly games and demos. I originally used them on my C64, then later on my Amiga.

One of my 5 1/4 drive, still running fine.

Boot selector

I had a switch to boot from the 5 1/4 drive. That was very handy, allowing me to boot directly from DF1.

I no longer have my original setup from 1991, but I recently installed a newer one into my A500. The new system allows switching with a key combination (long reset), so there is no physical switch. This means there is no need to make a hole in the Amiga for the switch.

External terminal for debugging

In France, we had an early national small internet network called 'Minitel'. It was linked with phone lines. You had to call servers via simplified numbers (e.g., 3614 RTEL2) and then browse BBS-like servers. The Minitel is also a passive terminal. I connected it to my Amiga and used it to display information and send commands to the Amiga. I was able to display memory and send debug commands.

One version of the french minitel.

Here are some source code I found. The date is 1996, so that was running on my A1200 (and not my A500). It is used for communication between Amiga and the terminal.

;
; Demo Minitel - Oriens/Les Shadock
;
;------------------------------------------------------------------
; PROGRAMME
;------------------------------------------------------------------
LogoMinitel:
Move.w #371,$dff032 ; 1200 bauds = 2981
Lea MTexte,a0
.bb1
Cmp.b #$ff,(a0) ; end of text
beq .end
Move.b (a0)+,d0
Bsr SendData
bra .bb1
.end: Rts
;------------------------------------------------------------------
MTexte: Dc.b $0c,$1b,$c1
Dc.b ' ',$60,$60,$60,$60,$60,$60,$60,$60
Dc.b $60,$60,$60,$60,$60,$60,$60,$60,$60,$60,$60,$60
Dc.b $60,$60,$60,$60,$60,$60,$60,$60,$60,$60,$60,$60
Dc.b $1b,$c7
Dc.b $1b,$cd
Dc.b $1b,$c8,' WELCOME ',$1b,$c9,$1b,$cc,$1b,$c1
dc.b 10,13
Dc.b $1b,$c7
dc.b $0e
Incbin xutils:utils/amitel/blabla/logo.mgfx
Dc.b $0f
dc.b $1b,$c5
dc.b $60,$60,$60,$60,$60,$60,$60,$60,$60
Dc.b 'Les Shadock in 1996 Oo'
dc.b $60,$60,$60,$60,$60,$60,$60,$60
Dc.b $0d,$0a
Dc.b ' \/'
Dc.b $ff
Even
;------------------------------------------------------------------
; FONCTIONS
;------------------------------------------------------------------
SendData:
; entree d0 = ascii data
Bsr Parite
Move.w d0,test
Bsr WaitSerie
Move.w d0,$dff030
Rts
;------------------------------------------------------------------
WaitSerie:
; wait transfer end
btst #12,$dff018
beq waitSerie
Rts
;------------------------------------------------------------------
Parite:
; input : d0 with data 7 bits
; output : d0 code to be sent to the minitel

And.w #$007f,d0 ; Get data 7 bits

Move.w d0,d1

; count in d3, bit number in d0 = d1
Moveq #0,d3
Move.w #7-1,d2
.bb Btst #0,d1
beq .bb1
addq #1,d3
.bb1 Lsr #1,d1
Dbra d2,.bb
Or.w #$0100,d0 ; first bit stop
; if odd bit count then need a second bit stop
Btst #0,d3
beq .no2stop
Or.w #$0080,d0 ; second bit stop
.no2stop
Rts
;------------------------------------------------------------------

On the unfinished Intro Ga prod of 1996, I found a text mentionning that there is some code if a minitel is plugged.

; UNFINISHED INTRO-GA PRESENTED BY
; LES SHADOCK
; FOR THE SATURNE 96
;
; CODE ORIENS
; GFX RAHOW
; MUSIC TEBIROD
;
; TRY TO PLUG YOUR MINITEL
; YOU WILL HAVE EXTRA PARTS!
;
; CONTACT US AT :
;
; GREETZ FLY TO :
;
; ABYSS APEX ARKHAM
; BOMB EKO EREMATION
; GODS KLOON MAD-DOGS
; NGC NHD OXYGENE
; SATURNE SCOOPEX
; SILICON SKARLA
; SPASM
; SYNDROME X-MEN
;
; AND ALL THE PEOPLE HERE...

In 2024 I bought a Minitel, so I'll be able to check if there is a special part when running that demo.

Step49 Step51