BeebWiki — ANDY

URL: https://beebwiki.mdfs.net/ANDY

Summary

Short page describing the ANDY paged-RAM area on the B+ and Master. Most of the content is already covered in paged-rom and memory-map; the new material worth adding is OSWORD-based access (&05/&06 with &FFFExxxx addresses), the B+ &A000-&AFFF shadow-display-manipulation trick, and the MOS-won’t-scan-it-for-languages rule.

Key technical claims

Per-machine size and location

Machine	Size	Location	Notes
B+	12 KB	&8000-&AFFF (top of shadow RAM, above the 20 KB shadow screen)	Paged in by setting top bit of &FE30 ROMSEL
Master 128	4 KB	&8000-&8FFF	Reserved for MOS system variables (see AllMem &8000-&8FFF workspace breakdown)

Master also exposes a separate 8 KB area as HAZEL at &C000-&DFFF (controlled by ACCCON Y — see shadow-ram).

Access methods

1. Paged-ROM space: write &FE30 with the top bit set (in addition to the bank number). Then &8000- reads/writes hit ANDY rather than the selected sideways bank.
2. OSWORD &05 — read byte using addresses formatted as &FFFExxxx.
3. OSWORD &06 — write byte using same address format.

The &FFFExxxx form is the I/O-processor “extended” addressing convention; the high &FFFE selects ANDY, low 16 bits select the offset.

B+ shadow-display trick

On the B+, code at &A000-&AFFF can directly manipulate shadow RAM display memory. The shadow display physically occupies the bottom 20 KB of the same RAM that ANDY pages into; mapping ANDY makes a 4 KB window into the top of shadow RAM addressable as normal memory.

Behaviour when shadow display is disabled is undocumented.

MOS behaviour

The MOS will not search for languages or service ROM images in this space.

Both B+ and Master. Implication: if you put code at &8000 in ANDY, you cannot enter it via *command dispatch — you must JSR directly with the right ROMSEL value already set.

Caveat

Original B+ documentation warned ANDY “may not be available in ‘other Acorn products’“. On the Master the address range and size differ (4 KB vs 12 KB) and the use is reserved for MOS — write-access from user code risks corrupting MOS state.

Cross-check against existing wiki

Claim	Wiki state	Resolution
ANDY on B+ = 12 KB at &8000-&AFFF	covered in paged-rom and memory-map	no change
ANDY on Master = 4 KB at &8000-&8FFF	covered	no change
OSWORD &05/&06 extended addressing	not documented	add to paged-rom
B+ &A000-&AFFF window onto shadow display	not documented	add to paged-rom + cross-link shadow-ram
MOS won’t scan ANDY for languages	not stated	add a one-liner to paged-rom

Filed into

• Updated: paged-rom (OSWORD &05/&06 access; B+ &A000-&AFFF shadow trick; MOS scan rule).

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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.