Fabritek MP-12 Loader

Kyle Owen kylevowen at gmail.com
Fri Dec 4 13:16:59 CST 2015

Alright, a lot has happened since 24 hours ago! In summary:

1. 6012 advances the tape one byte but doesn't change the accumulator.
2. 6015 reads the tape buffer and skips when ready. (Josh: you're correct;
the manual wasn't lying!)
3. 6052 and 6053 read the buttons and switch on the front panel of the
paper tape reader box. 6052 grabs the left four buttons in bits 8 through
11, and 6053 grabs the other five buttons and the switch in bits 4 through
11, with bits 2 and 3 permanently set for whatever reason.
4. With the knowledge of 1. and 2., I was able to get the format for
loading arbitrary sequential data into core. I'll explain the format below.
5. By taking DEC's BIN loader and stripping out all unnecessary things
(field switching, low speed reader support, etc.), I was able to make a BIN
loader for the Fabritek. It works!

The RIM format the Fabritek loader expects is something like this:
000 /leader
153 /5363 = JMP 7763, stored at 7755. Allows loader to continue running
after first deposit
1AB /ABCD = Starting address of program to load, minus one, stored at 7756
1WX /WXYZ = First word of program, stored at starting address specified
1BL /BLAH = more program data
000 /trailer

It's important that the starting address specified is one less than the
actual starting address, and the starting address can't be zero, as the
loader uses ISZ 7756 to increment the address prior to the first store;
7777 will result in zero, causing it to skip, causing it to terminate
prematurely. The loader keeps running until it overwrites the JMP 7763
instruction with a halt or something like that. I figure since this was
really meant to load a more intelligent loader, like the BIN loader, it
should overwrite that instruction with a JMP to the halt prior to the start
address of the BIN loader, so that you can just hit the run switch after
getting your BIN format tape loaded. So that's what I've done.

If you're not loading the BIN loader, but rather something else in lower
memory, it'd be a good habit to stop the reader during the trailer, since
any other characters (like 377, after the tape has left the reader) will be
loaded into core and muddle things up after your program. Might not be a
problem for a few words, but eventually, it'll probably do something weird
when it starts overwriting 7755...it'd probably take a while though. Best I
can tell, there is no way for it to detect the absence of tape in the
reader, so it'll happily go on reading 377.

Here's the BIN loader with the raw octal dump of the Fabritek RIM format
needed for loading it: http://pastebin.com/WiJDU1Bi

Doesn't look like these units had the real-time clock option installed.
Bummer! I was hoping to get a nice accurate tick for my alarm clock. Guess
I'll have to look at adding that myself.

Think that's all for now.


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