Cortex-M3 Processor (Reference Material)
UG0331 User Guide Revision 15.0 85
y specifies the condition switch for the third instruction in the IT block.
z specifies the condition switch for the fourth instruction in the IT block.
cond specifies the condition for the first instruction in the IT block. The condition switch for the second,
third and fourth instruction in the IT block can be either:
•
T
: Then. Applies the condition
cond
to the instruction.
•
E
: Else. Applies the inverse condition of
cond
to the instruction.
Note: It is possible to use
AL
(the always condition) for
cond
in an
IT
instruction. If this is done, all of the
instructions in the IT block must be unconditional, and each of x, y, and z must be
T
or omitted but not
E
.
3.6.9.3.2 Operation
The
IT
instruction makes up to four following instructions conditional. The conditions can be all the same,
or some of them can be the logical inverse of the others. The conditional instructions following the
IT
instruction form the IT block.
The instructions in the IT block, including any branches, must specify the condition in the
{cond}
part of
their syntax.
Note: Your assembler might be able to generate the required
IT
instructions for conditional instructions
automatically, so that you do not need to write them yourself. See your assembler documentation for
details.
A
BKPT
instruction in an IT block is always executed, even if its condition fails.
Exceptions can be taken between an
IT
instruction and the corresponding IT block, or within an IT block.
Such an exception results in entry to the appropriate exception handler, with suitable return information in
LR and stacked PSR.
Instructions designed for use for exception returns can be used as normal to return from the exception,
and execution of the IT block resumes correctly. This is the only way that a PC-modifying instruction is
permitted to branch to an instruction in an IT block.
3.6.9.3.3 Restrictions
The following instructions are not permitted in an IT block:
•IT
•
CBZ
and
CBNZ
•
CPSID
and
CPSIE
•
MOVS.N Rd, Rm
.
Other restrictions when using an IT block are:
• a branch or any instruction that modifies the PC must either be outside an IT block or must be the
last instruction inside the IT block. These are:
• ADD PC, PC, Rm
• MOV PC, Rm
•
B
,
BL
,
BX
,
BLX
• any
LDM
,
LDR
, or
POP
instruction that writes to the PC
•
TBB
and
TBH
• do not branch to any instruction inside an IT block, except when returning from an exception handler
• all conditional instructions except
Bcond
must be inside an IT block.
Bcond
can be either outside or
inside an IT block but has a larger branch range if it is inside one
• each instruction inside the IT block must specify a condition code suffix that is either the same or
logical inverse as for the other instructions in the block.
Note: Your assembler might place extra restrictions on the use of IT blocks, such as prohibiting the use of
assembler directives within them.
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