Where should the type information be 151

Andrew Reilly

code others).

The 2-bit combinations encode the usual meanings for arithmetic instructions: 0 = Zero, 1 = Low, 2 = High, 3 = Overflow. The interpretation does however depend on the CC-setting instruction, a source of frustration for novice asm370 programmers, and also emulator writers! Unsigned ("logical") subtraction in particular seems strange, since 2 the addition x + (2^32-x)). For floating-point comparisons 3 means "Unordered" (i.e. NaN involved).

Well, register renaming deals with most of those troubles, and unchecked condition codes need not even be computed (but watch out for exceptions and interrupts). The multiple CC fields of Power and PPC were in fact designed specifically to allow the compiler to schedule CC fields explicitly, like other registers. Many PPC opcodes also include a "record" bit that determines whether the CC should be set at all. On S-370 and friends, asm programmers and compiler code generators know which instructions leave the CC alone (for example, the Load-Address instr does aritmetic without affecting the CC), so some amount of clever instruction scheduling is possible here also. (When I code such sequences I'm always careful to include comments about "long-lived" condition codes, and where I expect to depend on them.) Michel.