I can't speak for all systems, but the ALTAIR and IMSAI use port $FF. It's decoded by a 74LS30 8 input NAND gate. I guess they could have used an inverter on the lsb (A8), but they didn't.
I use the SR and LED's on some of my monitor routines.
-LED's show which page is being tested during RAM test.
-When learning how to use the 8" disks, I programmed sw1 Floppy motor on, sw2 step in, sw3 step out, sw4 format the track, sw5 read the track to RAM (halt cpu to examine)
-LED show which track being formated
-Early program when testing the machine was to read switches and dump to LED's... (LED's are inverted)
(I think it was my 2nd program, the first was JMP $0000)
Finally, yes, I have used it like described. When testing software, I would write patterns to the LED's at strategic points, so I can tell if the program is reaching those "milestones".
I think the switches were also used when loading MS 4K basic. They would be used to select different hardware configurations, the LED's would indicate status of the loading from paper tape. There's a nice video of this on Youtube.
Date: Sat, 8 Feb 2014 09:31:25 -0800
Subject: [N8VEM-S100:2368] Switch Register
My system is old enough it has a switch register on it's Front Panel - I have a few questions.
- It's on port $FF - is this the correct port for the SR - for some reason I remember it being on $FE?
- I find it useful for debugging (writing bit patterns to the SR) - Does anyone else use one this way?
- Are there other uses for the SR?
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