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BBC Micro Model B

A 2 MHz 6502 home computer with 32 KiB RAM and the BBC Micro MOS/BASIC in ROM.

Manufacturer
Acorn Computers
Released
December 1981
Region
United Kingdom
Record status
approved
A cream BBC Micro on a blue cloth, its black keyboard crossed by a row of red function keys and the BBC owl badge at the top right of the case.

A BBC Micro Model B. The red keys along the top are the ten user-programmable function keys the machine was known for.

simon.inns · CC BY 2.0 · Wikimedia Commons

About this machine

The Model B's writable RAM and its MOS/BASIC ROM are both addressable system stores, but only the former belongs in the whole-system memory sum.

Configuration

Retail Model B

United Kingdom

Component Role Count Clock Capacity
MOS Technology 6502 Main CPU 1 2 MHz Nominal clock —
BBC Micro Video ULA GPU 1 unknown Nominal clock —
BBC Micro Model B RAM System memory 1 — 32 KiB shared
BBC Micro Model B ROM Read-only MOS and BASIC firmware. System memory 1 — 32 KiB shared

Figures for the whole machine

BBC Micro Model B: system figures
Metric Value Evidence Basis Method Source
Clock frequency cpu 2 MHz Confirmed Vendor-rated Nominal clock Stated by the vendor
Memory bandwidth memory 4.00 MB/s Confirmed Derived Theoretical peak Computed here from figures the vendor stated Divides bytes transferred by one shared-RAM access by the published memory cycle time. The result is an interface ceiling when a manual specifies nanoseconds per byte instead of a transfer rate. It is not measured application throughput: refresh, contention and wait states can consume cycles. Reported to 3 significant digits. That is the precision of the cycle time, because the bytes-per-cycle constant is exact.
Memory cycle time memory 250 ns Confirmed Vendor-rated Nominal cycle time Stated by the vendor One shared-RAM access; control alternates between the processor and CRTC.
Memory capacity whole-system 32 768 B Confirmed Derived Design capacity Computed here from figures the vendor stated Adds 1 memory-capacity figure obtained by design-capacity. Left out: 32 KiB of MOS and BASIC ROM (read-only firmware, not memory the machine can write to). A total is only as trustworthy as the decision about what belongs in it, so both the terms and the exclusions are named records.
3 documented absences
BBC Micro Model B: system figures: documented absences
Metric Value Evidence Basis Method Source
Clock frequency gpu unknown The documentation describes the ULA's timing functions without a standalone GPU frequency. Not stated Vendor-rated Nominal clock Stated by the vendor
Sustained FP40 rate whole-system unknown The non-IEEE BASIC format may be recorded only with a named reproducible benchmark run. Not stated Measured Benchmark run Published independently
Rated power consumption whole-system unknown The documented PSU rail capability is not a whole-system consumption rating. Not stated Vendor-rated Rated power Stated by the vendor

Figures for its components

1 documented absence
MOS Technology 6502: documented absences
Metric Value Evidence Basis Method Source
Peak FP32 rate cpu unknown The detailed hardware documentation does not identify a floating-point processor in the standard Model B. Not stated Vendor-rated Theoretical peak Stated by the vendor
BBC Micro Model B RAM
Metric Value Evidence Basis Method Source
Memory capacity memory 32 KiB Confirmed Vendor-rated Design capacity Stated by the vendor
BBC Micro Model B ROM
Metric Value Evidence Basis Method Source
Memory capacity memory 32 KiB Confirmed Vendor-rated Design capacity Stated by the vendor

Derived figures and comparisons

Computed by this project rather than read from a source, and recomputed from their inputs on every build.

4 000 000 B/s

Formula
bytes per memory cycle ÷ memory cycle time memory-bandwidth-from-cycle-time v1
Inputs
  • bbc-micro-model-b Memory cycle time: 250 ns
Constants
  • bytes transferred by one shared-RAM access: 1 The manual describes a data path for all eight data bits, so each cited RAM access transfers one byte.
Rounding
none , 3 significant digits

Divides bytes transferred by one shared-RAM access by the published memory cycle time. The result is an interface ceiling when a manual specifies nanoseconds per byte instead of a transfer rate. It is not measured application throughput: refresh, contention and wait states can consume cycles. Reported to 3 significant digits. That is the precision of the cycle time, because the bytes-per-cycle constant is exact.

32 768 B

Formula
a + b + … sum v1
Inputs
  • bbc-micro-ram Memory capacity: 32 KiB
Rounding
none , 5 significant digits

Adds 1 memory-capacity figure obtained by design-capacity. Left out: 32 KiB (read-only firmware, not memory the machine can write to). A total is only as trustworthy as the decision about what belongs in it, so both the terms and the exclusions are named records.

Sources