ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

A9.1.2 Null checking
ThumbEE
A null check is performed for all load/store instructions when they are executed in ThumbEE state. If the
value in the base register is zero, execution branches to the NullCheck handler at HandlerBase – 4.
For most load/store instructions, this is the only difference from normal Thumb operation. Exceptions to
this rule are described in this chapter.
Note
The null check examines the value in the base register, not any calculated value offset from the base
register.
If the base register is the SP or the PC, a zero value in the base register results in UNPREDICTABLE
behavior.
The instructions affected by null checking are:
all instructions whose mnemonic starts with LD, ST, VLD or VST
POP, PUSH, TBB, TBH, VPOP, and VPUSH.
For each of these instructions, the pseudocode shown in the Operation section uses the following function:
// NullCheckIfThumbEE()
// ====================
NullCheckIfThumbEE(integer n)
if CurrentInstrSet() == InstrSet_ThumbEE then
if n == 15 then
if IsZero(Align(PC,4)) then UNPREDICTABLE;
elsif n == 13 then
if IsZero(SP) then UNPREDICTABLE;
else
if IsZero(R[n]) then
LR = PC : ‘1’; // PC holds this instruction’s address plus 4
BranchWritePC(TEEHBR - 4);
EndOfInstruction();
return;
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. A9-3