## ptest - packed test

    SF = (dest & source) == 0
    CF = (~dest & source) == 0

The ptest instruction tests the 128 bit source value (second operand) versus the 128 bit destination value (an XMM register). The source can be an XMM register or a 128 bit memory location. There is also vptest on CPUs with AVX instructions which allows using 3 XMM registers or 2 XMM registers and a memory location which can simplify coding and which tests 256 bits versus 256 bits if you use YMM registers.

Assuming that you wish to test an XMM or YMM register to see if it is 0 in one instruction, you must be sure that the entire register was 0 before you use did the work with the register. For example if you load a non-zero double and then start using the register as a float it may have some extra non-zero bits and the test may fail to detect that the first 32 bits are 0. So use vzeroall originally and use the same size values in the registers and it will be a safe test.

            ptest   xmm0, xmm0      ; xmm0 & xmm0 == 0 if xmm0 == 0
                                    ; check ZF for equality
                                    ; use jz or jnz

Here are a few examples

            ptest   xmm0, xmm1      ; test 128 bits from xmm0 & xmm1
            ptest   xmm0, [x]       ; test 128 bits from xmm0 & x
                                    ; x is an array of floats
            vptest  xmm0, xmm15     ; test 128 bits from xmm0 & xmm15
            vptest  ymm0, [x]       ; test 256 bits from ymm0 & x
            vptest  ymm0, [rsi]     ; test 256 bits from ymm0 & [rsi]
                                    ; rsi contains the address of an array

### flags: ZF CF

ZF is set to 0 if the and of the 2 operands yields 0. This means that they have no 1 bits in the same positions. This can happen when they are both 0.

CF is set to 0 if the and of the destination inverted and the source yields 0. This means that wherever the destination has 0's the source also has 0's. Inverting the destination would convert all its 0's to 1's, so any 1 in the result comes from a 0 in the destination and a 1 in the source. If the result is 0, then 1's in the destination line up with 0's in the source. I can't suggest why this is useful.