Three blocks move and nothing else changes. Branches take 0x60, subroutines
take 0x70, and the ALU moves up into the 0x10 block the two of them used to
share. Order within each block is preserved exactly - this relocates them,
it does not rethink them.
WHAT IT BUYS IS AN EMPTY 0x00 TO 0x0F. Program Memory that was never
written, or a load that stopped part way and left zeroes in its tail, used
to read as a long run of ADDs: the machine carried on through them, arrived
somewhere unpredictable, and whatever broke there was a long way from the
byte that caused it. Now it faults where it is met:
Fault: 0x00 at Program Address 0x0004 is not an instruction.
That is the address of the byte after the last real instruction, which is
the difference between a diagnosis and a search. Reserving the whole nibble
rather than just 0x00 means a run into blank memory faults wherever it
starts rather than only when it lands on the right byte. runOffTest records
it, and the block is left empty for whatever turns out to want it.
The other half is room: branches and subroutines had filled 0x10 to 0x1F
between them, so a service return that keeps Q and DP3 had nowhere to sit
next to its family. It has 0x76 waiting now.
Five places wrote an opcode down that the scripted remap did not reach, and
four of them were found by tests rather than by looking:
- secondPass.c lists which opcodes take an address, and firstPass.c knows
SWI by number. Missing those made XOR read as a branch.
- Asm.asm knows SWI by number too, being the other assembler. Missing it
made the native and host assemblers disagree byte for byte, which is
exactly the check that exists to catch a thing known in two places.
- loaderTest.asm carries a hand written payload, and its RETI was 0x19. To
the assembler those are numbers and to the program they are data, so
nothing but running it could notice. It says so in a comment now.
- The Assembler Manual prints the bytes hello.asm assembles to, and two of
them were branches.
The monitor's recorded disassembly moved by exactly the bytes it should:
18 became 72 wherever SWI appears, with SETD and INIB untouched and every
disassembled line still reading the same.
A token is classified after its quotes have been stripped, so a string
literal reading "ADD" looked exactly like the ADD instruction and was
assembled as one. It failed with "attempting to assemble outside the
Program Segment", a message about a mistake nobody had made.
The literal and label checks were already guarded against strings and the
instruction check was not. Mnemonics match without regard to case, so
"or" and "and" were caught by this too, and those are ordinary enough
words to want in a message.
Third of its family, after a string beginning with '0' being read as a
malformed number and a string in the Program Segment being silently
discarded. All three have the same root.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Anachronaut
Anachronaut