CODEWITHSUNDEEP
assemblyfloating-pointx86masm32
Shadow
Shadow

Reputation: 4006

compare instruction not working correctly

I am comparing some input received from the user with some numbers, and the same jump is taken regardless of the compare.

User input is defined as so:

.data?
    userInput REAL4 ?

And when I compare it I use:

cmp userInput, 18200
jge _tb0    ;This jump is always taken

However I have tested with userInput being less than, equal, and great than 18200 but it still jumps to the same label.

I have just started learning assembly from this tutorial, which has been great so far, but doesn't have a lot of information and I am having trouble finding a good, detailed tutorial online so that I can learn what I am doing wrong.

UPDATE:

After reading a bit on using the FPU to do floating point math, I came up with the following code for variables:

.data?
    userInput real4 ?

.const
    tb0_max real4 18200.00

and the following code for comparing:

fld userInput
fcom tb0_max
jge _tb0

However I still get the same result as before, I jump to _tb0 regardless of the value given as user input

UPDATE 2:

After reading this on comparing with the FPU, I came across the following code which should, according to the article, allow me to move the compare states from the FPU to the CPU so that I can use a regular CPU compare instruction:

fld userInput
fcom tb0_max
fstsw ax
fwait
sahf
jge _tb0

However I am still having the same issue, the compare always result as the userInput being less than the constant 18200.00 regardless of userInput

Upvotes: 2

Views: 299

Answers (2)

Fifoernik
Fifoernik

Reputation: 9899

Since you want to compare your single precission float (userInput) with an integer (18200) you could just temporarily store it as an integer and then perform the comparison:

.data?
userInput   REAL4 ?
tempInteger dd ?

...

fld   userInput
fistp tempInteger
cmp   tempInteger, 18200
jge   _tb0

Upvotes: 1

Alexey Frunze
Alexey Frunze

Reputation: 62048

cmp itself works correctly. But you aren't using it correctly. REAL4 is a floating point type. cmp works with integer operands only. Either use integers or learn how to compare floating point values.

Upvotes: 2

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