Monday, December 5, 2016

RC2014/LL Showing ROM/RAM switching, 64k!

So I thought I'd just make a quick post to show how I know that the ROM swap hardware in my RC2014/LL is working. I'll walk through the procedure here...

First, I booted up the RC2014 as usual, which drops me into BASIC.  From there I wrote this short program:
10 FOR A = 0 to 8192
20 B = PEEK( A ) 
What this does is goes through the lowest 8 kbytes of memory, reads (peek) and then writes (poke) the byte it finds right back to the place it got it from.   For example:
  1. Read from memory address 0
  2. Write to memory address 0
  3. Switch to the next memory address, repeat 
Which seems stupid, that it does nothing.  But if you remember from a previous post, when the ROM is enabled, it's only enabled for reads.  Writes still happen to the RAM in the same addresses.  So the above program copies the BASIC ROM into RAM.

Next, we switch off the ROM by doing this:
 OUT 0,1
The lowest bit, '1' turns off the ROM, and switches to the RAM for read operations.  To confirm we're working out of the RAM, we can poke a 0 in somewhere down there.

POKE 0,0
So we can see here that memory address 0 has been changed from 243 to 0.

I also went one step further and yanked the ROM board out of my RC2014.  This is not recommended. ;)  But it did continue to run without any problems... until I really tried to muck things up, for fun:

I wrote a program to clear RAM, and then ran it in BASIC.  Here is the listing:

And here's what happened when i typed 'run'.  It completely locked up after it printed the '8'. Starting at memory address 0x0008 is the text output routine, so it cleared out the boot code before that (0x0000-0x0007), erased the beginning of the print handler, then went to print out something to the screen, and crashed.  Well, I assume it crashed. It got an invalid opcode and who knows what it's actually doing. The code there used to be a "jump" which is 3 bytes.  Instead it got a NOP, which does nothing, (0x00)  and then two garbage bytes which map to something incorrect. Boom!