Importance of Q(Saturation Flag) in ARM

Question

I want to understand the importance of Q flag in ARM Processor. I know there are certain instructions like QADD,QSUB etc.

But I need to understand this with some examples which will clarify the concept.

Answer 1

This is explained in the "ARM Architecture Reference Manual" (ARM DDI 0100E):

Bit[27] of the CPSR is a sticky overflow flag, also known as the Q flag. This flag is set to 1 if any of the following occurs:

• Saturation of the addition result in a QADD or QDADD instruction
• Saturation of the subtraction result in a QSUB or QDSUB instruction
• Saturation of the doubling intermediate result in a QDADD or QDSUB instruction
• Signed overflow during an SMLA<x><y> or SMLAW<y> instruction

The Q flag is sticky in that once it has been set to 1, it is not affected by whether subsequent calculations saturate and/or overflow. Its intended usage is:

1. Use an MSR CPSR_f,#0 instruction to clear the Q flag (this also clears the condition code flags).
2. Peform a sequence of calculations.
3. Use an MRS Rn,CPSR instruction to read the CPSR, then test the value of the Q flag. If it is still 0, none of the above types of saturation or overflow occured during step 2. Otherwise, at least one instance of stauration or overflow occured.

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An example:

mov     r2,#0x70000000
qadd    r3,r2,r2

0x70000000 + 0x70000000 would become 0xE0000000, but since qadd is saturating, the result is saturated to 0x7FFFFFFF (the largest positive 32-bit integer) and the Q flag is set.