NAUG Ch12 — Memory

Holmes & Dickens, The New Advanced User Guide, pp.162-171. Compact (10pp) but high-leverage — defines the memory map all other chapters reference.

Key facts captured

• 6502 address space is 64 KB. Common to all BBC machines:
  • &0000-&2FFF (12 KB) — user RAM, MOS variables.
  • &3000-&7FFF (20 KB) — screen RAM (max). The portion not used by current MODE is available to the user.
  • &8000-&BFFF (16 KB) — paged ROM/RAM (sideways).
  • &C000-&FBFF — MOS ROM.
  • &FC00-&FCFF — FRED (1 MHz bus expansion / cartridges on Master).
  • &FD00-&FDFF — JIM (1 MHz bus expansion / cartridges on Master).
  • &FE00-&FEFF — SHEILA (memory-mapped I/O for all hardware: 6845, ULA, VIAs, paging registers).
  • &FF00-&FFFF — MOS ROM (vectors etc.).
• ROM paging register at SHEILA &FE30. Low 4 bits (PR0-PR3) select one of 16 paged ROM banks at &8000-&BFFF. Bit 7 is machine-specific:
  • Master: bit 7 set → 4 KB of RAM (“ANDY”) swapped in at &8000-&8FFF (used by MOS during graphics).
  • B+: bit 7 set → 12 KB of RAM at &8000-&AFFF.
• Test sideways RAM: OSBYTE &44 returns X with bits 0-3 set for SWR banks 4-7. Available on B+/Master only.
• ACCCON register at SHEILA &FE34 controls shadow RAM, HAZEL filing-system RAM, Tube selection, cartridge vs 1 MHz bus. Layout differs between B+ and Master:
  • B+: only S bit (single shadow select; combined VDU-write + display).
  • Master: bits 0=D, 1=E, 2=X, 3=Y, 4=ITU, 5=IFJ, 6=TST, 7=IRR. See shadow-ram.
• Shadow RAM: B+ and Master have an extra 20 KB bank at &3000-&7FFF independent of main memory. When selected (*shadow, OSBYTE &72, or MODE 128-135), screen lives in shadow and &3000-&7FFF in main memory is free for user code/data.
• HAZEL (Master only): 8 KB filing-system RAM at &C000-&DFFF overlaying MOS VDU driver. Switched by ACCCON Y bit. Must be switched back before next OSWRCH or the VDU driver vanishes mid-call.
• OSBYTE &84 returns the bottom of display RAM (i.e. HIMEM for non-shadow) — or &8000 if shadow mode is in use. OSBYTE &85 same but for a given mode (X = mode).
• Master direct-access OSBYTEs for double-buffered animation:
  • &6C — select which &3000-&7FFF bank the CPU sees (X=0 main / 1 shadow).
  • &70 — which bank the VDU driver writes to (X=0 default / 1 main / 2 shadow).
  • &71 — which bank the 6845 reads from for display (X=0 default / 1 main / 2 shadow).
  • Combined: write next frame into the bank that isn’t currently displayed, then flip &71.
• OSBYTE &FE: machine ID (X = 0 Electron / 1 B+ / &40 Model A / &80 Model B). Master uses this byte differently.

Critical do-not warning (§12.3 p168)

Never write &FE30 directly from user code. The MOS keeps track of the active paged ROM; an unsynchronised write can swap the current language ROM out and crash the machine instantly. Use the relevant OSBYTEs / service calls (Ch17 ingest, pending). The MOS itself changes &FE30 often — assume the value is volatile across any OS call.

Filed into

• memory-map — 64KB layout with machine-specific extras.
• paged-rom — sideways ROM/RAM mechanics, paging register, ANDY.
• shadow-ram — shadow RAM + ACCCON, including Master vs B+ differences and double-buffering technique.
• Plus the page-3 / zero-page VDU workspace mentioned in §13.2 — folded into memory-map references.

Open follow-ups

• HAZEL precise use cases — chapter says “filing systems”; full picture needs Ch16 ingest.
• ROM service call protocol (&FE30 write side-effects) — Ch17 ingest.

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