PALM (Program All Logic in Microcode)

The PALM processor (Program All Logic in Microcode) was a 16-bit CPU developed by IBM. It was used in the IBM 5100 Portable Computer line, a predecessor to the IBM PC. The PALM processor was also used in other IBM products as an embedded controller.

IBM referred to PALM as a microprocessor. In IBMs terminology this meant a processor that executes microcode to implement a higher-level instruction set, rather than its conventional definition of a CPU. The PALM processor was a circuit board containing 13 bipolar gate arrays packaged in square metal cans, 3 conventional transistor-transistor logic ICs in dual in-line packages, and 1 round metal can part.

The PALM was used to implement an emulator, which in turn coulrd run machine instructions written for other machines; this is how IBM System/360 APL ran on the 5100 machine.

The PALM has a 16-bit data bus, with two additional parity bits. PALM can directly address 64kByte of memory. The IBM 5100 could be configured with up to 64+ kB (APL + BASIC) Executable ROS (ROM), and up to 64kB of RAM. A bank switching scheme was used to extend the address space.