Double compare-and-exchange

By: Howard Chu (, March 25, 2008 5:18 pm
Room: Moderated Discussions
Linus Torvalds ( on 3/25/08 wrote:
>So there are two "forms" of the double CAS: the one that
>operates on two contiguous words, and the one that can work
>with two different addresses. x86 (and many others) have
>the former, almost nobody has the latter (I think some of
>the more obscure versions of the m68k had it).

M68020 and up worked on any two words. CAS and CAS2. Not really obscure, though I suppose the plain 68000 was still the most-produced chip. Considering how many successful multiprocessor supercomputers were built on the 68020 instruction set, I think it was actually pretty well-known. (Not to mention all the single-processor workstations and servers...)

CAS Dc,Du,
CAS2 Dc1:Dc2,Du1:Du2,(Rn1):(Rn2)

CAS compares the effective address operand (ea) to the compare operand (Dc). If the operands are equal, the update operand (Du) is written to the effective address operand.; otherwise the effective address operand is written to the compare operand (Dc).

CAS2 compares memory operand 1 (Rn1) to compare operand 1 (Dc1). If the operands are equal, it compares memory operand 2 (Rn2) to compare operand 2 (Dc2). If these operands are also equal, it writes the update operands (Du1 and Du2) to the memory operands (Rn1 and Rn2). If either comparison fails, it writes the memory operands to the compare operands. A single RMW sequence is used for the memory accesses.

The Dx operands indicate that the value resides in the specified data register. The indicates that the value resides in memory and can be specified using any of the available memory-alterable addressing modes (register indirect, with displacement, index, absolute, whatever...)
The (Rn1) operands indicate that the value's address is in the specified (data or address) register.
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