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AllMem: BBC System Memory Map (Ripley / Harston)

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AllMem — BBC System Memory Map

Author: Jon Ripley, updated by J.G. Harston Edition used: 18-Jun-2016 File: raw/AllMem.txt Scope: A complete byte-level catalogue of MOS workspace use across the BBC A/B/B+, Electron, Master 128, Master ET, and Master Compact. Covers zero page, pages 1-3, buffer pages 8-D, sideways ROM areas, FRED/JIM/SHEILA, and the MOS jumpblock.

Summary

This is the single most useful low-level reference for “what owns this byte?” — finer-grained than the NAUG memory chapter (which is more discursive) and authoritative on BBC/Electron/Master differences. Per-byte coverage, with per-machine variants called out side-by-side. Particularly strong on:

  • Zero page subdivisions (Econet/NMI/CFS/VDU partitioning) — more precise than NAUG.
  • The full OSBYTE-variables table at &236-&28F (every fxNNN ↔ address ↔ purpose mapping).
  • Extended vector table at &0D9F-&0DEF.
  • Sideways-ROM workspace conventions for the &0DF0-&0DFF private workspace high-byte register.
  • Electron-specific deltas (ULA register copies, ROM manager workspace).

Key technical claims (with citations to file lines)

Citations are line numbers in raw/AllMem.txt.

Zero page partitioning (lines 13-130)

  • &0000-&008F Language workspace — full 144 bytes, not just &00-&6F (which is what NAUG implies when listing the “USER &70-&8F” carve-out).
  • &0090-&009F Econet private workspace.
  • &00A0-&00A7 Current NMI owner’s workspace. &00A0 conventionally stores previous NMI owner ID.
  • &00A8-&00AF Transient command space (NOT generic “OS scratch” — used by *command invocations).
  • &00B0-&00BF Filing system scratch space.
  • &00C0-&00CF Filing system workspace.
  • &00D0-&00E1 VDU workspace.
  • &00E2-&00E3 CFS/RFS private workspace.
  • &00E4-&00FF General MOS workspace.

VDU zero page detail (lines 68-88)

  • &00D0 VDU Status (= OSBYTE &75). Bits: 0=printer, 1=scroll-disabled, 2=paged, 3=software-scroll, 4=shadow (Master only), 5=VDU 5, 6=cursor split, 7=VDU disabled.
  • &00D1 Graphics pixel mask.
  • &00D2 Text colour OR mask. &00D3 Text colour EOR mask.
  • &00D4 Graphics colour OR mask. &00D5 Graphics colour EOR mask.
  • &00D6-&00D7 Graphics character cell.
  • &00D8-&00D9 Top scan line.
  • &00DA-&00DF Temporary workspace.
  • &00E0-&00E1 BBC/Electron: Row multiplication; Master: General workspace.

Critical OS workspace at &00E4-&00FF (lines 106-130)

  • &00EC Internal key number (last). &00ED Internal key number (first/penultimate).
  • &00EE Internal key of char ignored by OSBYTE &79, AND the 1 MHz bus paging-register RAM copy (dual-purpose).
  • &00EF / &00F0 / &00F1 OSBYTE/OSWORD A / X / Y reg values during a call.
  • &00F2-&00F3 OS text pointer (star commands, filenames).
  • &00F4 Currently selected ROM.
  • &00F5 Current PHROM or RFS ROM number.
  • &00F6-&00F7 PHROM/RFSROM/OSRDSC pointer.
  • &00F8-&00F9 BBC/Electron unused; Master: Soft key expansion pointer.
  • &00FA-&00FB General OS workspace, used by buffer access code in interrupts.
  • &00FC Interrupt Temp A reg store.
  • &00FD-&00FE Error message pointer, initially set to language version string (NOT “post-BRK return pointer” — it’s the address of the error text following a BRK).
  • &00FF Escape flag (bit 7).

OS vectors at &0200-&0235 (lines 149-177)

Full 27-vector table. All present in our existing calls page; AllMem confirms.

OSBYTE variables &0236-&028F (lines 179-287)

Each address &023N maps to fxNNN where NNN = address − &023 mapped to OSBYTE number. The mapping is &236 = fx166 = OS-var &A6, then sequential.

Examples (used to verify the wiki’s “direct read” table):

  • &024A fx186 &BA ROM active last BRK.
  • &0261 fx209 &D1 Speech suppression status.
  • &0262 fx210 &D2 Sound suppression status.
  • &0269 fx217 &D9 Screen lines since last page.
  • &027C fx236 &EC Output stream character destination (set with FX3).
  • &027D fx237 &ED Cursor key status (set with FX4).
  • &027E fx238 &EE BBC/Electron unused; Master: keypad base.
  • &0242 fx178 &B2 Keyboard semaphore (BBC/Master) / undefined (Electron).

Page 2 misc workspace (lines 289-369)

  • &0290 VDU vertical adjust (*TV first parameter).
  • &0291 Interlace toggle (*TV second parameter).
  • &0292-&0296 TIME value 1, &0297-&029B TIME value 2 (5-byte dual-copy for atomic read).
  • &029C-&02A0 OSWORD 3/4 countdown timer.
  • &02A1-&02B0 Paged ROM type table.
  • &02B1-&02B2 INKEY countdown.
  • &02B3 / &02B4 / &02B5 OSWORD 0 max line length / min char / max char.
  • &02B6-&02BE ADC channel value tables (BBC). Electron uses &02F7-&02FF instead.
  • &02BF-&02C8 Event enable flags.
  • Buffer busy / start / end indexes differ BBC vs Master vs Electron — table at lines 326-336.

Page 3 VDU workspace (lines 378-445)

  • &0300-&0307 Current graphics window (4 × 16-bit words: left col, bottom row, right col, top row — pixels).
  • &0308-&030B Current text window (4 bytes: left, bottom, right, top — character cells).
  • &030C-&030F Graphics origin (external coords).
  • &0310-&0313 Current graphics cursor (external).
  • &0314-&0317 Old graphics cursor (external).
  • &0318 / &0319 Current text cursor X / Y.
  • &031A Line within graphics cell of cursor.
  • &031B Graphics workspace / VDU queue.
  • &031F-&0323 VDU queue (5 bytes).
  • &0324-&0327 Current graphics cursor (internal coords).
  • &0328-&032F Bitmap read from screen by OSBYTE 135.
  • &0330-&0349 Graphics workspace.
  • &034A-&034B Text cursor address for 6845.
  • &034C-&034D Text window width in bytes.
  • &034E High byte of bottom of screen memory.
  • &034F Bytes per character for current mode.
  • &0350-&0351 Screen display start address for 6845 (the OS’s view of R12/R13).
  • &0352-&0353 Bytes per screen row.
  • &0354 Screen memory size high byte.
  • &0355 Current screen mode.
  • &0356 Memory map type: 0=20K, 1=16K, 2=10K, 3=8K, 4=1K.
  • &0357-&035A Foreground/background text/graphics colours.
  • &035B / &035C Foreground/background plot mode.
  • &035D-&035E General VDU jump vector.
  • &035F Cursor start register previous setting.
  • &0360 Number of logical colours − 1.
  • &0361 Pixels per byte − 1 (zero if text-only mode).
  • &0362 / &0363 Leftmost / rightmost pixel colour mask.
  • &0364 / &0365 Text input cursor X / Y.
  • &0366-&036E BBC/Electron: teletext output cursor char + font explosion flags + font location MSBs for char groups 32-63, 64-95, … 224-255. Master: VDU 23,16 setting, VDU 23,6 dot pattern, ECF state, foreground/background graphics colour.
  • &036F-&037E Palette for colours 0-15.
  • &037F Unused (used by some shadow-RAM systems).

Pages 8-9 buffers (lines 494-578)

  • &0800-&083F Sound workspace.
  • &0840-&087F Sound channel buffers (4 × 16-byte channels).
  • &0880-&08BF Printer buffer (DISTINCT from sound area).
  • &08C0-&08FF Envelopes 1-4 (13 bytes each + 3 spare).
  • &0900-&09BF RS423 output buffer.
  • &09C0-&09FF Speech buffer.
  • &0900-&09FF overlap: CFS/RFS BPUT sequential output buffer (mutually exclusive with RS423/Speech).
  • &0900-&09BF Envelopes 5-16 (same layout as 1-4, when used).
  • &0A00-&0AFF Cassette/RS423 input buffer (NOT used by LOAD).

Page B / Page C / Page D (lines 586-748)

  • &0B00-&0B10 Soft key pointers (17 bytes = ptr per fkey 0-15 + sentinel).
  • &0B11-&0BFF Soft key strings.
  • Master variant: &0B00-&0BFF Econet workspace (station, FS station, FS network, print server type string).
  • &0C00-&0CFF BBC/Electron: Font for ASCII 128-159. Master: Econet workspace.
  • &0D00-&0D5F NMI handler code + workspace. Claim via service-call 12 (OSBYTE 143,12,255). MUST place RTI at &0D00 if you own the workspace but don’t handle hardware NMIs.
  • Econet uses &0D20-&0D5F for scout/acknowledge packets when active.
  • &0D60-&0D67 Econet workspace.
  • &0D68-&0D7F Master Econet / Electron expansion / Electron ROM manager workspace.
  • &0D80-&0D91 Econet-over-other-hardware workspace.
  • &0D92-&0D9E Mouse / trackerball workspace.
  • &0D9F-&0DEF Extended vector table — every standard vector mirrored as 3 bytes (address low, address high, ROM number). E.g. &0DAE-&0DB0 XBYTEV.
  • &0DF0-&0DFF Sideways ROMs private workspace high-byte register. ROM x has byte at &0DF0+x. Conventionally MSB of private workspace pointer. Bit 7 disables the ROM (workspace at &8000-&FFFF = ROM area = invalid).

Master Andy workspace (lines 820-829)

&8000-&8FFF (Andy private RAM, ACCCON-bit selected):

  • &8000-&8020 Soft key string start addresses (16 × low byte + sentinel + 16 × high byte + sentinel).
  • &8022-&83FF Soft key string data.
  • &8400-&87FF VDU driver workspace.
  • &8800-&88FF VDU variables.
  • &8900-&8FFF Font definition, chars 32-255.

Master Hazel workspace (lines 832-886)

&C000-&DBFF Sideways ROM shared / private workspace (claim via service call &21 for shared, &22 for private). Strictly: only filing systems can claim shared workspace here (via FSC 6 — vectors changing). Private workspace is per-ROM.

  • &DC00-&DCFF OSCLI string buffer.
  • &DD00-&DEFF Transient command workspace.
  • &DF00 Current filing system number.
  • &DF01 Active filing system number.
  • &DF02 Library filing system number.
  • &DF03 ROM number of current filing system.
  • &DF04-&DF05 Transient command line address.
  • &DF06-&DFC1 Up to 17 filing system info blocks (11 bytes each + terminator).
  • &DFC2-&DFDD *LIST/*TYPE/*BUILD/*APPEND/*MOVE state.
  • &DFDE-&DFFF Unused.

Master DFS OSWORD &7F bug (line 856-858)

The Master DFS OSWORD &7F writes to page &C2 regardless of who owns the shared workspace. Documented bug, worth noting if rolling custom Master ROMs.

Page FE — SHEILA (lines 927-948)

Confirms our existing memory-map table. AllMem additions:

  • &FE18-&FE1F Station ID / NMI Control (BBC B / B+) vs ADC (Master) — note the Master ADC is at &FE18, not &FEC0 (which holds the BBC’s ADC).
  • &FE20-&FE2F Video ULA plus border control plus network NMI control.
  • &FE24-&FE27 Disk control (Master).
  • &FE80-&FE9F: 8271 FDC (B) / 1770 FDC + control (B+) / Videodisk SCSI (Master).
  • &FEC0-&FEDF: ADC (B/B+) / “Network interface (not 6854 ADLC)“.

Page FF — MOS entry jumpblock (lines 951-981)

Full list of public entry points &FFB3-&FFF9 plus hardware vectors &FFFA-&FFFF (NMIV, RESETV, IRQV). Master adds OSWRSC (&FFB3) for writing a byte to screen.

Filed into

This ingest produces no new wiki pages — it’s a reference cross-check. The audit findings are listed in allmem-cross-reference-report (filing this back depends on user approval — see report).

Pages whose claims were validated against this source:

Contradictions found are listed below for human review.

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.

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