ASSEMBLY NOT AND NEG FULL
ASSEMBLY NOT AND NEG CODE

ITTEE refers to If-Then-Then-Else-Else (next 4 instructions are conditional)Įach instruction inside the IT block must specify a condition suffix that is either the same or logical inverse.ITTE refers to If-Then-Then-Else (next 3 instructions are conditional).ITE refers to If-Then-Else (next 2 instructions are conditional).ITT refers to If-Then-Then (next 2 instructions are conditional).IT refers to If-Then (next instruction is conditional).The structure of the IT instruction is “IF-Then-(Else)” and the syntax is a construct of the two letters T and E: z specifies the condition switch for the fourth instruction in the IT block.y specifies the condition switch for the third instruction in the IT block.x specifies the condition switch for the second instruction in the IT block.cond specifies the condition for the first instruction in the IT block.Some ARM processor versions support the “IT” instruction that allows up to 4 instructions to be executed conditionally in Thumb state. Specifically, the Thumb version which allows conditional execution (Thumb-2). In the Instruction Set chapter we talked about the fact that there are different Thumb versions.

As a result, the second ADDLT is not executed and r0 remains unmodified. Now we have V = 0 and N = 0 which results in LT condition to fail. That’s why second CMP clears out Negative bit (because 3 – 3 = 0, no need to set the negative flag) and sets the Zero flag (Z = 1). Before we execute second CMP, our r0 = 3.

ASSEMBLY NOT AND NEG FULL

Subsequently, the ADDLT instruction is executed because LT condition is full filled when V != N (values of overflow and negative bits in the CPSR are different).

ASSEMBLY NOT AND NEG CODE

The first CMP instruction in the code above triggers Negative bit to be set (2 – 3 = -1) indicating that the value in r0 is Lower Than number 3. Negative bit get's set to 1 */Īddlt r0, r0, #1 /* increasing r0 IF it was determined that it is smaller (lower than) number 3 */Ĭmp r0, #3 /* comparing r0 to number 3 again. Mov r0, #2 /* setting up initial variable */Ĭmp r0, #3 /* comparing r0 to number 3.

We can use the following piece of code to look into a practical use case of conditions where we perform conditional addition.global main The following table lists the available condition codes, their meanings, and the status of the flags that are tested. If the first number was bigger, we would have a Greater Than condition and in the opposite case – Lower Than. There are more conditions, like Lower or Equal (LE), Greater or Equal (GE) and so on. For example, when we compare two numbers and they turn out to be equal, we trigger the Zero bit (Z = 1), because under the hood the following happens: a – b = 0. Those bits change from time to time based on the outcome of some instructions. The condition is described as the state of a specific bit in the CPSR register. We use conditions for controlling the program’s flow during it’s runtime usually by making jumps (branches) or executing some instruction only when a condition is met. We already briefly touched the conditions’ topic while discussing the CPSR register.