The trace log

The trace log records the machine state before every instruction executed: the master cycle count, PC, all main and alternate registers, IX, IY, SP, the word on top of the stack, I, R, the interrupt mode and both interrupt flip-flops, the memory page mapped in each of the eight slots and which of them held ROM, the memory the instruction was fetched from, the decoded flags and the raw opcode bytes. It is a ring buffer of 10 000 entries, so it always holds the last 10 000 instructions.

• F2, the toolbar F2: Trace button (its indicator turns green), or Debug ▸ Trace ▸ Enable Trace switches it on.
• Debug ▸ Trace ▸ Clear Trace empties it.
• F3, the F3: Export Trace button, or Debug ▸ Trace ▸ Export Trace… writes it to a text file, one instruction per line. ROM= is a hex mask of the slots that held ROM: bit 0 is slot 0, so ROM=03 means slots 0 and 1. FETCH= names the memory the instruction itself was fetched from, which a slot's page number cannot say when an overlay answers; it is one of the tags in the table below.
• --trace on the command line starts with it enabled.

F2 and F3 are the defaults and can be rebound — see Changing the keys.

FETCH= tag	Memory
P0E	RAM page 0x0E (two hex digits, 00 to DF)
ROM0 to ROM3	ROM image 0 to 3
BOOT	boot ROM
MFROM / MFRAM	Multiface ROM / RAM
DIVROM	DivMMC ROM
DIVRAM3	DivMMC RAM bank 0 to 15 (decimal)
ALTROM0 / ALTROM1	alternate ROM: 0 = 128K, 1 = 48K
L2P12	Layer 2 read mapping, Layer 2 page 0x12
CFG17	config mode, NR 0x04 bank 0x17
FLOAT	floating bus (no memory answers)

The spelling of a tag is stable: capital letters and digits only, never changed between versions.

An interrupt or NMI fetches no instruction; its line names the memory the interrupted instruction will come from.

The trace is what makes "how did we get here?" answerable after a breakpoint fires. It is also a prerequisite for stepping backwards.