What is the difference between test and cmp in compilation? Extra points for getting it right!!

The friend on the first floor said that it was a bit troublesome, but it was actually easier to explain.

1.Difference in meaning:

CMP belongs to the category of addition and subtraction operations, while Test belongs to the category of logical operations.

2.Conceptual differences.

CMP is similar to SUB, that is, the source operand is subtracted from the destination operand, but SUB will send the result to the destination operand, while CMP will not send the destination operand;

Test is similar to AND in that it performs a bitwise "AND" operation on two operands, but AND sends the result to the destination operand, while test does not.

3.Differences in usage.

CMP is mainly used to compare the relationship between two numbers, and can be compared between signed and unsigned numbers. Equal numbers affect the ZF flag, unsigned numbers affect the CF flag, and signed numbers affect the SF and OF signs. Generally, program jumps are set according to the influence of flags, that is, the corresponding program fragments are completed according to different conditions, similar to the if-else structure of high-level languages.

All conditional transfer orders can be used after this order.

test is usually used to check if certain bits are 1, but you don't want to change the operand. For example, to check whether there is one bit of 1 in bit 6 and bit 2 in al, you can use the following command:

test al, 01000100b, if all two bits are 0If the value of zf is 1, otherwise it is clear, then the jump set according to the flag can only be jz or jnz

The similarity between these two instructions is that neither of them affects the operand, but only reflects the result of the operation through some bits of the flag register.

text x,y

The main function is that x and y are combined, and the result is not sent back, but affects the flag bit.

cmp x,y

The function is x-y, does not send back the result, and also affects the flag bit....

The most liked downstairs said that it was very good, but I personally feel that it is a bit too much for beginners The subject asks test and cmp

What's the difference, I'll start with that.

The function of CMP: to determine whether two numbers are equal Example: cmp ax ax is equal, zf is 1, if it is unequal, zf is 0

The (common) function of test: to determine whether the register content is zero Example: test ah ah if ah is o then zf is 1 if ah is not 0 zf is 0

If it's the subject's question, the above is enough. Then the next thing is for the students who want to read the above and want to read the following.

zf (zero flag) Chinese: zero flager Function: reflect the operation result When the operation result is zero, zf is 1, and when the operation result is 1, zf is 0 (this is easy to understand, because it is a zero flagor, it means that there is zero as 1).

CMP Instruction Principle (two operands are subtracted, but no value is returned, only the flag is modified): e.g. cmp ax bx

If ax-bx is equal to 0, then zf is 1

The principle of the test directive (two operands are operated with, but the value is not returned, only the flag bit is modified): e.g. test ah ah

Then in this instruction, the result is always 0 and zf is 1