Cortex-A8 Technical Reference Manual - ARM Information Center

5.2.1 Return stack predictions
Program Flow Prediction
Note
Instructions with the S suffix are not predicted. They are typically used to return from
exceptions and have side effects that can change privilege mode and security state.
• ADD(4) with PC destination in Thumb state
• MOV(3) with PC destination in Thumb state
• CPY with PC destination in Thumb state
• CZB in Thumb state
• TBB/TBH in Thumb state
Note
In Thumb state, a branch that is normally encoded as unconditional can be conditioned by
inclusion in an If-Then-Else (ITE) block. Then it is treated as a normal conditional branch.
• HB (ThumbEE state only)
• HBP (ThumbEE state only)
• HBL (ThumbEE state only)
• HBLP (ThumbEE state only).
The HBL and HBLP instructions act as function calls and push the return address onto the
return stack.
The return stack stores the address and the ARM or Thumb state of the instruction after a
function-call type branch instruction. This address is equal to the link register value stored in
r14.
The following instructions cause a return stack push if predicted:
• BL immediate
• BLX(1) immediate
• BLX(2) register
• HBL (ThumbEE state)
• HBLP (ThumbEE state).
The following instructions cause a return stack pop if predicted:
• BX r14
• MOV pc, r14
• LDM r13, {…pc}
• LDR pc, [r13]
• LDM r9, {..pc} (ThumbEE state only)
• LDR pc, [r9] (ThumbEE state only).
The LDR instruction can use any of the addressing modes, as long as r13 is the base register.
Additionally, in ThumbEE state you can also use r9 as a stack pointer so the LDR and LDM
instructions with pc as a destination and r9 as a base register are also treated as a return stack
pop.
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