ARM Assembler - How do I use CMP, BLT and BGT?

Question

Quick question for you guys, in my loop I need to use CMP , BLT and BGT to compare some values. How would use said instructions in the following loop?

I'm trying to use BGT , BLT and CMP as I need them to make my application work. The trouble is I have no idea how to use them. If I wanted to use CMP to compare r6, with r4 and put the difference into r7, how would I do this? The same question if I wanted to use BLT if r7 is less than 0, how would I do this?

  BGT ??????? ; branch if greater than 5
  CMP ???????? ; compare r6 with r4 , put difference into r7
  BLT ???????? ;branch if r7 is less than 0
  BGT ???????? ;branch if r7 is greater than 0

Here's my entire loop:

LoopStart

  BL WaitBUT1
  BL readTemp
  BL checkTemp
  BGT ??????? ; branch if greater than 5
  BL errorVal
  CMP ???????? ; compare r6 with r4 , put difference into r7
  BLT ???????? ;branch if r7 is less than 0
  BL FanOn
  BL errorLedOn
  BL systemLedOn
  BL heaterOn
  BGT ???????? ;branch if r7 is greater than 0
  BL FanOff
  BL errorLedOff
  BL systemLedOff
  BL heaterOff
  BL WaitBUT2
  BL FanOff
  BL errorLedOff
  BL systemLedOff
  BL heaterOff

  B LoopStart

Answer 1

You cannot do a conditional branch without first setting the condition register somehow. This can be done with cmp or by adding s to most instructions. Check out the ARM assembly documentation for details. Quick example:

Branch if r0 greater than 5:

cmp r0, #5 ;Performs r0-5 and sets condition register
bgt label_foo ;Branches to label_foo if condition register is set to GT

Compare r6 with r4 , put difference into r7, branch if r7 < 0:

subs r7, r6, r4 ;Performs r7 = r6 - r4 and sets condition register
blt label_bar ;Branches to label_bar if r7 < 0 (in which case r6 < r4)

Answer 2

If I wanted to use CMP to compare r6, with r4 and put the difference into r7, how would I do this?

subs r7, r6, r4    /* r7 ← r6 - r4 */

The same question if I wanted to use BLT if r7 is less than 0, how would I do this?

bmi _exit          /* branch if r7 < 0 */

BMI (minus/negative) When N is enabled (N is 1) where N is a flag that will be enabled if the result of the instruction yields a negative number. Disabled otherwise.

Why subS instead of sub? Because S is an optional suffix that when is specified, the condition flags (like N) are updated on the result of the operation.

Regards.