mirror of https://github.com/dschmenk/PLASMA.git synced 2024-12-22 02:31:12 +00:00

Location	Use
$00 - $05	unused by Plasma
$06 - $07	`SRC` - Temporarily used in the `memcpy` routine as the source address. (In PLASMA for the Apple I, this is called `freemem` and is used during initialization to pass the start of the heap to the command interpreter.)
$07 - $08	`DST` - Temporarily used in the `memcpy` routine as the destination address.
$09 - $BD	unused by PLASMA
$BE - $BF	`ESGUARD` - Stack guard? Seems to be unused directly.
$C0 - $CF	`ESTKL` Evaluation stack low byte; together with `ESTKH`, forms a 16-bit wide stack indexed by the `X` register
$D0 - $DF	`ESTKH` Evaluation stack high byte
$E0	`VMZP` - ?? I think this is supposed to mark the start of the zero page locations used by the VM, but it's unclear since many addresses before are also used.
$E0 - E1	`IFP` - Interpreter frame pointer (initialized to `#$8000` on the Apple I, not sure on Apple ][.)
$E2 - E3	`PP` - Param pointer
$E4	`IPY` - Instruction pointer offset (`Y` register) temporary storage
$E5	`ESP` - Stack pointer (`X` register) temporary storage
$E6	`JMPTMP` - Always `#$4C`, a `JMP` instruction, allowing `JSR JMPTMP` to dispatch to the location in `TMP`, usually as a subroutine.
$E7 - $E8	`TMP` - A 16-bit temporary register.
$E9	`DVSIGN` (Apple I) - Sign of dividend after division
$EA	`DVSIGN` (Apple ][ 64K) - Sign of dividend after division
$EF - FF	`PAGE0` - Interpreter bytecode inner loop. The following locations are part of the loop.
$EF	`DROP` - Drops one frame of the evaluation stack, then interprets the next opcode.
$F0	`NEXTOP` - Interprets the next opcode.
$F1	`FETCHOP` - Fetch the current opcode.
$F2 - $F3	`IP` - Instruction pointer for PLASMA interpreter
$F4 - $F6	part of zero page routine
$F7	`OPIDX` - location of the lower byte of the address in `JMP (OPTBL)`, used to dispatch into the interpreter
$F8	`OPPAGE` - location of the high byte of the address in `JMP (OPTBL)`.
$F9 - $FF	part of the zero page routine, but currently unused