Escape

Pressing the ESCAPE key sets a system-wide ESCAPE condition. Most OS routines check between operations and bail back to BASIC if it’s set, producing the “Escape” error. Held in MOS workspace until explicitly cleared.

OSBYTE summary

OSBYTE	Function	Common use
&7C (124)	Clear condition (no effects)	Manually clear after handling an escape
&7D (125)	Set condition (programmatic ESCAPE)	Service ROM aborts something
&7E (126)	Acknowledge + clear effects	Standard “what BASIC does on ESCAPE”
&C8 (200)	R/W escape-disable + BREAK behaviour	Games (disable both)
&DC (220)	R/W ESCAPE character (default &1B)	Remap ESCAPE to another key
&E5 (229)	R/W ESCAPE key status	Treat ESCAPE as plain ASCII
&E6 (230)	R/W ESCAPE effects flag	Lighter &7E that only clears the flag

OSBYTE &C8 — disable / BREAK control

Packs two unrelated things:

bit 0: 0 = ESCAPE works,  1 = ESCAPE disabled (only OSBYTE &7D can set it)
bit 1-7: 0 = preserve memory on BREAK,  non-zero = clear memory on BREAK

*FX 200,n	Effect
n=0	Normal (default)
n=1	ESCAPE disabled, memory preserved
n=2	ESCAPE works, memory cleared on BREAK
n=3	ESCAPE disabled, memory cleared on BREAK

*FX 200,1 is the canonical “game launcher” setting — survives soft BREAK with memory intact, ESCAPE doesn’t kill your game.

What OSBYTE &7E tears down

When OSBYTE &E6 flag = 0 (default), acknowledging ESCAPE (OSBYTE &7E) also:

• Closes any open *EXEC file.
• Purges all buffers (input + output, sound, printer, RS423).
• Resets VDU paging counter (lines since last halt).
• Clears the VDU queue (&323).
• Cancels any current soft-key expansion.
• Stops any sound currently playing.

That’s a lot. Set OSBYTE &E6, X=&FF to make &7E only clear the condition without touching anything else.

Patterns

Game — disable ESCAPE completely

LDA #&C8 : LDX #1 : LDY #0 : JSR &FFF4    ; ESCAPE disabled

Or treat ESCAPE as plain ASCII (so it enters the buffer but doesn’t abort):

LDA #&E5 : LDX #&FF : LDY #0 : JSR &FFF4

The user can still press it; it just doesn’t cause an abort. Read it from the key buffer like any other key.

Game — handle ESCAPE manually

Hook event 6 (events):

LDA #&0E : LDX #6 : JSR &FFF4         ; enable event 6
LDA #handler MOD 256 : STA &220       ; install EVNTV
LDA #handler DIV 256 : STA &221

In the handler: set a flag, return. The main loop checks the flag and does what you want (pause, quit-to-menu, etc.). Then call OSBYTE &7E to acknowledge cleanly.

Remap ESCAPE to a different key

LDA #&DC : LDX #ASC"Q" : LDY #0 : JSR &FFF4    ; ESCAPE is now 'Q'

Or to make TAB the ESCAPE key:

LDA #&DC : LDX #9 : LDY #0 : JSR &FFF4         ; ESCAPE is now TAB (ASCII 9)

Light acknowledge (preserve sound/buffers)

LDA #&E6 : LDX #&FF : LDY #0 : JSR &FFF4   ; disable side effects of &7E
LDA #&7E : JSR &FFF4                       ; clear condition only
LDA #&E6 : LDX #0 : LDY #0 : JSR &FFF4     ; restore default for next time

See also

• osbyte — All ESCAPE-related entries (&7C-&7E, &C8, &DC, &E5, &E6).
• events — Event 6 = ESCAPE detected.
• brk — BREAK is a different thing (direct reset line).
• keyboard — TAB / ESCAPE character defaults.
• errors — error 17 (Escape) is what BRKV sees when the MOS auto-converts an ESCAPE flag to a BRK.

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