BBC Micro Performance Wiki

NAUG Ch10 — Escape

3 min read

NAUG Ch10 — Escape-related calls

Holmes & Dickens, The New Advanced User Guide, pp.154-156. Short chapter (3 pages) covering the ESCAPE condition — how it’s triggered, suppressed, cleared, and what state it tears down.

ESCAPE in a nutshell

  • The ESCAPE key (by default ASCII &1B) raises a system-wide ESCAPE condition.
  • Most OS routines check the condition between operations and abort if it’s set, returning to the calling BASIC program with an “Escape” error.
  • Event 6 (events) fires when ESCAPE is detected if event 6 is enabled.
  • The condition stays set until explicitly cleared.

OSBYTE calls

OSBYTEFunctionNotes
&7C (124)Clear ESCAPE condition (no side effects)X=&FF returned if a condition existed and was cleared, X=0 if none. Tells Tube if active.
&7D (125)Set ESCAPE condition (simulate key press)No event generated. Useful from a service ROM to programmatically abort.
&7E (126)Acknowledge ESCAPE + effectsClears condition AND flushes buffers, closes *EXEC, resets VDU paging counter + queue, cancels soft-key expansion, stops any sound. X=&FF if cleared, X=0 if none.
&C8 (200)R/W escape disable + BREAK actionSee bits below.
&DC (220)R/W ESCAPE characterDefault &1B (27). *FX 220,65 makes ‘A’ the ESCAPE key.
&E5 (229)R/W ESCAPE key status0 = normal, ≠0 = treat ESCAPE as plain ASCII (no escape event).
&E6 (230)R/W ESCAPE effects flag0 = OSBYTE &7E does all the side effects above; ≠0 = &7E only clears the condition.

OSBYTE &C8 (200) — escape disable + BREAK action

This OSBYTE packs two unrelated functions:

BitsMeaning
00 = normal ESCAPE; 1 = ESCAPE disabled unless caused by OSBYTE &7D
1-70 = normal BREAK action; non-zero = clear memory on BREAK

Set with OSBYTE &C8, X=value, Y=&00 (or via *FX 200,value):

  • *FX 200,0 — ESCAPE works, BREAK preserves memory (default).
  • *FX 200,1 — ESCAPE disabled, BREAK preserves memory.
  • *FX 200,2 — ESCAPE works, BREAK clears memory.
  • *FX 200,3 — ESCAPE disabled, BREAK clears memory.

Practical patterns

Game-style disable

For games that handle their own input and don’t want ESCAPE to abort:

LDA #&C8 : LDX #1 : LDY #0 : JSR &FFF4    ; disable ESCAPE key
LDA #&E5 : LDX #&FF : LDY #0 : JSR &FFF4  ; treat ESCAPE as plain ASCII

Or, less destructively, leave ESCAPE active but trap event 6 (events).

Custom ESCAPE key

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

Suppress effects of a programmatic ESCAPE

Sometimes you want ESCAPE to interrupt a long-running thing but not tear down the VDU queue / sound / soft keys:

LDA #&E6 : LDX #&FF : LDY #0 : JSR &FFF4   ; ESCAPE effects disabled
; ... later ...
LDA #&7E : JSR &FFF4                       ; acknowledge — only clears condition

Side-effects of OSBYTE &7E when effects enabled

NAUG §10.7 p149 — when OSBYTE &E6 flag = 0:

  • Any open *EXEC file is closed.
  • All buffers purged (including input buffer).
  • VDU paging counter (lines since last halt) reset.
  • VDU queue reset.
  • Any current soft-key expansion is cleared.
  • Any sound being produced is terminated.

When &E6 flag ≠ 0, only the condition itself is cleared — none of the above.

Filed into

  • escape — Compact escape-handling reference (new page).
  • Updates: osbyte entries &7C/&7D/&7E/&C8/&DC/&E5/&E6 cross-linked.
  • Updates: events — event 6 cross-link.

Open follow-ups

  • None — this is a thin chapter; everything captured.

This wiki is curated by Claude following the LLM-Wiki methodology — a human curates source documents, the LLM compiles structured cross-linked markdown. Content may contain errors, omissions, or stale claims. For authoritative information refer to the original source documents in the bbc-documents GitHub archive.

Graph View

Backlinks