G'evening. I'm doing a lab work on Assembly. I have to compare 3 numbers -8, -2 and 11 to get the greatest. I translated them in an additional code and represented them in hexadecimal notation. So I get wrong answer because -2 = FE > 11 = B. The question is how can I compare these numbers correctly? I use nasm, x86.

CodePudding user response：

The compare instruction, as well as almost every arithmetic instructions will modify a set of flags in a status register.

The flags are modified independent of the intent of an instruction -- e.g. the Parity Flag or Auxiliary Flag is modified regardless of the (rare) need to check the status of that flag.

When -2 is compared with 11, or 11 is subtracted from -2, the processor actually does not know that -2 is negative; instead 0xfe - 0x0b is computed, which does not overflow, thus CarryFlag == 0. Also because the result is not zero, ZeroFlag == 0 as well. And because 0xFE has a different sign than 0x0b, OverflowFlag == 0 too.

The different combinations of all these OF,ZF,CF map to higher level abstractions, such as unsigned <= unsigned, any != any, signed > signed. In Intel Architecture 'Below / Above' refer to unsigned compares and 'Greater / Less' refer to signed comparison -- also the same set of flags can refer to multiple abstractions, such as JC (see http://unixwiz.net/techtips/x86-jumps.html)

The subset of signed comparisons you probably need are then either JG or it's complement JNG, which stands for Jump Not Greater, which is equal to JLE or Jump Less or Equal.

In addition to conditional branch, x64 instruction set supports conditional set/move of a register and armv7 allows conditional execution of most instructions and armv8 allows conditional execution of some interesting sequences of instructions (like a = b iff LessThan, else a = c 1).