MMIX Instruction Set

SAVE and UNSAVE

SAVE $X,0
UNSAVE $Z

Specification:

SAVE: Condition X ≥ rG

u($X) ← context

rL ← 0.

UNSAVE: context ← u($Z)

Timing:

υ + 20μ

Description:

Saves and restore the process state in the register stack.

SAVE | UNSAVE:

"save process state" | "restore process state" (the Y field must be 0, and so must the Z field of SAVE, the X field of UNSAVE.) The SAVE instruction stores all registers and special registers that might affect the computation of the currently running process. First the current local registers $0, $1, . . . , $(L - 1) are pushed down as in PUSHGO $255, and L is set to zero. Then the current global registers $G, $(G+1), . . . , $255 are placed above them in the register stack; finally rB, rD, rE, rH, rJ, rM, rR, rP, rW, rX, rY, and rZ are placed at the very top, followed by registers rG and rA packed into eight bytes: The address of the topmost octabyte is then placed in register $X, which must be a global register. (This instruction is interruptible. If an interrupt occurs while the registers are being saved, we will have α = β = γ in the ring of local registers; thus rO will equal rS and rL will be zero. The interrupt handler essentially has a new register stack, starting on top of the partially saved context.) Immediately after a SAVE the values of rO and rS are equal to the location of the first byte following the stack just saved. The current register stack is effectively empty at this point; thus one shouldn't do a POP until this context or some other context has been unsaved. The UNSAVE instruction goes the other way, restoring all the registers when given an address in register $Z that was returned by a previous SAVE. Immediately after an UNSAVE the values of rO and rS will be equal. Like SAVE, this instruction is interruptible. The operating system uses SAVE and UNSAVE to switch context between different processes. It can also use UNSAVE to establish suitable initial values of rO and rS. But a user program that knows what it is doing can in fact allocate its own register stack or stacks and do its own process switching. Caution: UNSAVE is destructive, in the sense that a program can't reliably UNSAVE twice from the same saved context. Once an UNSAVE has been done, further operations are likely to change the memory record of what was saved. Moreover, an interrupt during the middle of an UNSAVE may have already clobbered some of the data in memory before the UNSAVE has completely finished, although the data will appear properly in all registers.