31-bit - Transition

Transition

The transition was tricky: assembly language programmers had been using the spare byte at the top of addresses for flags for almost twenty years. IBM chose to support two forms of addressing to minimize the pain: if the most significant bit (bit 0) of a 32-bit address was on, the next 31 bits were interpreted as the virtual address. If the most significant bit was off, then only the lower 24 bits were treated as the address (just as with pre-XA systems). Thus programs could continue using the seven low-order bits of the top byte for other purposes as long as they left the top bit off. The only programs requiring modification were those that set the top (leftmost) bit of a word containing an address. This also affected address comparisons: The leftmost bit of a word is also interpreted as a sign-bit, indicating a negative number if bit 0 is on. Programs that use signed arithmetic comparison instructions could get reversed results. Two equivalent addresses could be compared as non-equal if one of them had the sign bit turned on even if the remaining bits were identical. Fortunately, most of this was invisible to programmers using high-level languages like COBOL or FORTRAN, and IBM aided the transition with dual mode hardware for a period of time.

Certain machine instructions in this 31-bit addressing mode alter the addressing mode bit as a possibly intentional side effect. For example, the original subroutine call instruction BAL stores certain status information in the top byte of the return address. A BAS instruction was added to support 31-bit return addresses. BAS, and its register-register equivalent, BASR, Branch and Store Register, was part of the instruction set of the System/360 Model 67, which was the only System/360 model to support addresses longer than 24 bits. These instructions were maintained, but were modified and extended for 31-bit addressing. Additional instructions in support of 31-bit addressing included several call/return instructions which also effected an addressing mode change (e.g. Branch and Set Mode, Branch and Store and Set Mode).