IBM 5150  -  Cassette BASIC

This is the BASIC that is built into the IBM 5150 motherboard.

The 5150 motherboard will boot to cassette BASIC if it cannot find a disk that it can boot from.

Cassette BASIC resides in motherboard chips U29, U30, U31 and U32. Those four chips contain the BASIC and nothing else.

IBM possibly named it 'cassette BASIC' because it supports the cassette port only, e.g. you cannot write/read BASIC files to/from floppy disks (or hard disk drives).
If you require disk support, then use the BASIC or BASICA included in IBM PC DOS.


When cassette BASIC starts, one of the things displayed is the version number.
In a 5150, it will be either version C1.00 or the later C1.10
The 'C' portion is an indicator that the BASIC is the 'cassette' version.

RAM usage

Although cassette BASIC runs from ROM, it uses RAM (to hold your program, for variables, etc.)
If cassette BASIC is run, its RAM usage is as follows:
16KB-64KB motherboard  ---> Whatever RAM is fitted on the motherboard (either 16KB, 32KB, 48KB, or 64KB)
64KB-256KB motherboard  ---> 64 KB


In regard to this built-in BASIC, 'cassette BASIC' is the term used in the Guide to Operations manuals for the 5150.
In the Technical Reference manuals, 'cassette BASIC' is also used, but 'resident BASIC' has been used in some places instead.


Part numbers

The IBM part number is printed on top of the chip.
Following are some sets that I have seen.

  Set #1 Set #2 Set #3 Set #4
U32 5700043 5700043 6359113 5000023
U31 5700035 5700035 6359112 5000022
U30 5700027 5000020 6359111 5000021
U29 5700019 5700019 6359109 5000019
  C1.00     C1.10


If one or more of the BASIC chips fail, the resulting symptom varies according to the revision of BIOS (BIOS resides in chip U33) that is fitted.

At power on, you see the flashing cursor as expected.
A few seconds later, one long beep followed by one short beep. About 15 seconds later, a single short beep.
No further progress. Even after a few minutes, a flashing cursor is still on screen.
No indication that points to the BASIC chips.
The floppy light comes on and stays on.
Then, inserting a boot floppy does nothing (no boot attempt made).
U29 failure: "F600 ROM" displayed
U30 failure: "F800 ROM" displayed
U31 failure: "FA00 ROM" displayed
U32 failure: "FC00 ROM" displayed