Memory Map

64 KB visible to the 6502 across all BBC family machines. The common layout:

&FFFF +--------------------------------------+
      | MOS ROM (vectors)              &FF00 | 256 B
&FF00 +--------------------------------------+
      | SHEILA -- memory-mapped I/O    &FE00 | 256 B
&FE00 +--------------------------------------+
      | JIM -- 1 MHz bus / cartridge   &FD00 | 256 B
&FD00 +--------------------------------------+
      | FRED -- 1 MHz bus / cartridge  &FC00 | 256 B
&FC00 +--------------------------------------+
      | MOS ROM                              | ~15 KB
&C000 +--------------------------------------+
      | Paged ROM / Sideways RAM (1 of 16)   | 16 KB
&8000 +--------------------------------------+
      | Screen RAM (mode-dependent base)     | up to 20 KB
&3000 +--------------------------------------+
      | User RAM + MOS workspace + lang/BASIC| 12 KB
&0000 +--------------------------------------+

Source: NAUG §12.1 p155-156.

Sub-regions in user RAM (&0000-&2FFF)

Range	Use
&0000-&00FF	Zero page. User zero page at &70-&8F (Ch2 §2.10). MOS uses most of the rest.
&0100-&01FF	Stack
&0200-&02FF	OS vectors, OS variables
&0300-&03FF	VDU driver workspace + filing system buffers (see os-workspace)
&0400-&07FF	Currently-selected language (BASIC) workspace + OS workspace
&0800-&08FF	Sound / printer / keyboard buffers (default)
&0900-&09FF	RS423/cassette buffer
&0A00-&0AFF	Keyboard buffer
&0B00-&0BFF	User-defined function-key strings
&0C00-&0CFF	User-defined character definitions (ASCII 224-255)
&0D00-&0DFF	NMI handler (1 page)
&0E00-…	DFS / other filing system workspace, then user program (PAGE)

PAGE (BASIC) / OSHWM (MOS) marks the start of user program memory. On Model B with DFS, that’s typically &1900. Master with HAZEL filing-system RAM enabled, that’s &0E00 (DFS workspace moved into HAZEL).

SHEILA address allocations (&FE00-&FEFF)

Range	Device	Notes
&FE00-&FE07	crtc-6845 6845 CRTC (address @ &FE00, data @ &FE01)	All machines except Electron
&FE08-&FE0F	6850 ACIA (serial)	All BBC; Electron only if serial expansion fitted
&FE10-&FE17	Serial ULA	All machines
&FE18	Econet station number	Master / Compact
&FE18-&FE1A	µPD7002 ADC	Electron only (ADC moves here on Electron)
&FE20-&FE2F	video-ula Video ULA (control @ &FE20, palette @ &FE21)	All machines
&FE24	Floppy disc control register	Master / Compact
&FE28-&FE2F	WD1770 FDC	Master / Compact
&FE30-&FE33	paged-rom ROM paging register	All machines
&FE34-&FE37	shadow-ram ACCCON register	B+ / Master / Compact
&FE38	INTOFF — Network NMI disable	B+ / Master
&FE3C	INTON — Network NMI enable	B+ / Master
&FE40-&FE5F	system-via System VIA	All machines
&FE60-&FE7F	user-via User/Printer VIA	All machines
&FE80-&FE9F	8271 FDC (Model B) / 1770 FDC (B+/Master)	—
&FEA0-&FEBF	68B54 Econet ADLC	If Econet fitted
&FEC0-&FEDF	µPD7002 ADC	BBC; on Master also Econet interface
&FEE0-&FEFF	tube-ula Tube ULA	All except Electron

Most peripheral registers repeat (mirror) across their 32-byte block — only certain low addresses are functional.

Per-MODE screen base

See modes. Range is always inside &3000-&7FFF; the base depends on screen size: &3000 for 20 KB modes, &4000 for 16 KB (MODE 3), &5800 for 10 KB (MODE 4/5), &6000 for 8 KB (MODE 6), &7C00 for 1 KB (MODE 7).

Machine extras beyond the common map

• Model B: as above. 16 KB MOS, 16 KB sideways for languages.
• B+: adds 20 KB shadow RAM and 12 KB paged RAM (bit 7 of &FE30 selects). Total RAM 64 KB on B+, 128 KB on B+128.
• Master 128: 20 KB shadow RAM, ANDY (4 KB private at &8000-&8FFF, selected via ROMSEL &FE30 bit 7), HAZEL (8 KB filing-system RAM at &C000-&DFFF, selected via ACCCON Y bit), 4 KB private MOS workspace, plus 16 sideways slots (some RAM, some ROM).
• Master Compact: similar with adjusted ROM map.
• Electron: Model-B-equivalent map; no shadow RAM, no Tube, video ULA combined with CPU clock generation. ADC moves to &FE18-&FE1A. Electron ULA functions at &FE00-&FE0F.

See paged-rom and shadow-ram for the details.

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