APPENDIX A. X86 APPENDIX A. X86
This branch of cryptography is fast-paced and very politically charged. Most designs are secret; a
majority of military encryptions systems in use today are based on LFSRs. In fact, most Cray computers
(Cray 1, Cray X-MP, Cray Y-MP) have a rather curious instruction generally known as “population count.”
It counts the 1 bits in a register and can be used both to efficiently calculate the Hamming distance
between two binary words and to implement a vectorized version of a LFSR. I’ve heard this called the
canonical NSA instruction, demanded by almost all computer contracts.[Sch94]POPFrestore flags from the stack (AKAEFLAGS register)
PUSHA(M) pushes the values of the (R|E)AX, (R|E)CX, (R|E)DX, (R|E)BX, (R|E)BP, (R|E)SI, (R|E)DI registers to the stack.
PUSHFpush flags (AKAEFLAGS register)
RCL(M) rotate left via CF flag:
7 6 5 4 3 2 1 0 CFCF 7 6 5 4 3 2 1 0RCR(M) rotate right via CF flag:
CF 7 6 5 4 3 2 1 07 6 5 4 3 2 1 0 CFROL/ROR(M) cyclic shift
ROL: rotate left:7 6 5 4 3 2 1 0CF 7 6 5 4 3 2 1 0ROR: rotate right:7 6 5 4 3 2 1 07 6 5 4 3 2 1 0 CFDespite the fact that almost allCPUs have these instructions, there are no corresponding operations in C/C++, so the
compilers of thesePLs usually do not generate these instructions.For the programmer’s convenience, at leastMSVChas the pseudofunctions (compiler intrinsics)_rotl()and_rotr()^8 , which
are translated by the compiler directly to these instructions.SALArithmetic shift left, synonymous toSHL
SARArithmetic shift right
7 6 5 4 3 2 1 07 6 5 4 3 2 1 0 CFHence, the sign bit always stays at the place of theMSB.SETccop: load 1 to operand (byte only) if the condition is true or zero otherwise. The condition codes are the same as in
the Jcc instructions (A.6.2 on page 886).
STC(M) set CF flag
(^8) MSDN