The Legac-e Information Site
Legac-e - Assembler for non-Assembler Programmers
Register usage convention
There are four types of Register, but the emphasis here is on General Purpose Registers (GPRs) of which there are
16 per processor.
Each executing program uses the same GPRs and is responsibie for saving (SAVE) and restoring (RETURN) them so as not
to compromise other programs.
The illustration opposite shows the function of Registers 0, 1, 13, 14, and 15 during the LINK/CALL interface between programs.
Whilst Register 12 is described as a Base Register, in truth the Base could be any register between 3 and 12, and some IBM
routines use alternatives.
The next page describes the GPRS.