IBM Cassette BASIC
The IBM Cassette BASIC is the Microsoft BASIC supplied in ROM on every IBM PC (5150) and IBM PC XT (5160) motherboard. It lives in sockets U29, U30, U31, U32 on the 5150 board, and is automatically launched by the BIOS when no boot disk is found in drive A.
History
Cassette BASIC was written by Microsoft for IBM as part of the original PC project. The same ROM image was supplied unchanged on the 5160 XT and on early 5170 AT machines. It was dropped from the IBM PCjr and the later PS/2 line.
ROM Layout
The 5150 motherboard carries Cassette BASIC in four sockets:
- U29, U30, U31, U32 — Cassette BASIC ROM (BIOS lives separately in U33).
Disk Variants
Three different BASIC builds shipped from IBM, layered on top of Cassette BASIC:
| Name | Form | Notes |
|---|---|---|
| Cassette BASIC | ROM (U29-U32) | Cassette I/O only; no disk; no advanced graphics |
| Disk BASIC (BASIC.COM) | Loaded from PC DOS, calls into ROM | Adds disk I/O on top of Cassette BASIC |
| Advanced BASIC (BASICA.COM) | Loaded from PC DOS, calls into ROM | Adds CGA graphics, sound, and event trapping |
A 5150 booted without any disk drops straight into Cassette BASIC. Disk and Advanced BASIC are not standalone — both rely on the ROM image and will not run on a clone with no Cassette BASIC ROM. This is why GW-BASIC was developed for the clone market: it is a self-contained equivalent that does not need IBM's ROM.
Common Faults
- "BASIC ROM" missing or corrupt — POST passes but the BASIC banner is garbled or BASIC will not start. Reseat U29-U32, then swap each ROM with a known-good donor to identify the failed chip.
References
- Cassette BASIC on the 5150, minuszerodegrees.net.