IT
The IT
(If-Then) instruction makes a single following instruction (the IT block) conditional. The conditional instruction must be from a restricted set of 16-bit instructions.
Syntax
IT
cond
where:
cond
- specifies the condition for the following instruction.
Deprecated syntax
IT
{
{x
{y
}}} {z
}
cond
where:
x
- specifies the condition switch for the second instruction in the IT block.
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 switches for the second, third, and fourth instructions 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.
Usage
The IT block can contain between two and four conditional instructions, where the conditions can be all the same, or some of them can be the logical inverse of the others, but this is deprecated in Arm®v8.
The conditional instruction (including branches, but excluding the
BKPT
instruction) must specify the condition in
the {
part
of its syntax.cond
}
You are not required to write IT
instructions
in your code, because the assembler generates them for you automatically
according to the conditions specified on the following instructions. However,
if you do write IT
instructions, the
assembler validates the conditions specified in the IT
instructions
against the conditions specified in the following instructions.
Writing the IT
instructions ensures
that you consider the placing of conditional instructions, and the
choice of conditions, in the design of your code.
When assembling to A32 code, the assembler performs the same checks,
but does not generate any IT
instructions.
With the exception of CMP
, CMN
, and TST
, the
16-bit instructions that normally affect the condition flags, do not affect them
when used inside an IT block.
A BKPT
instruction in an IT block is always
executed, so it does not require a condition in the {
} part of its syntax. The
IT block continues from the next instruction. Using a cond
BKPT
or
HLT
instruction inside an IT block is deprecated.
Note
You can use anIT
block for unconditional
instructions by using the AL
condition.Conditional branches inside an IT block have a longer branch range than those outside the IT block.
Restrictions
The following instructions are not permitted in an IT block:
IT
.CBZ
andCBNZ
.TBB
andTBH
.CPS
,CPSID
andCPSIE
.SETEND
.
Other restrictions when using an IT block are:
- A branch or any instruction that modifies the PC is only permitted in an IT block if it is the last instruction in the block.
- You cannot branch to any instruction in an IT block, unless when returning from an exception handler.
- You cannot use any assembler directives in an IT block.
Note
armasm shows a diagnostic message when any of these instructions are used in anIT
block.Using any instruction not listed in the following table in an IT block is deprecated. Also, any explicit reference to R15 (the PC) in the IT block is deprecated.
Table 13-9 Permitted instructions inside an IT block
16-bit instruction | When deprecated |
---|---|
MOV , MVN |
When R or
R is the
PC |
LDR , LDRB ,
LDRH , LDRSB ,
LDRSH |
For PC-relative forms |
STR , STRB ,
STRH |
- |
ADD , ADC ,
RSB , SBC ,
SUB |
ADD SP, SP, #imm or
SUB SP, SP, #imm or when
R ,
R or
R is the
PC |
CMP , CMN |
When R or
R is the PC |
MUL |
- |
ASR , LSL ,
LSR , ROR |
- |
AND , BIC ,
EOR , ORR ,
TST |
- |
BX , BLX |
When R is the PC |
Condition flags
This instruction does not change the flags.
Exceptions
Exceptions can occur between an IT
instruction
and the corresponding IT block, or within an IT block. This exception
results in entry to the appropriate exception handler, with suitable
return information in LR and SPSR.
Instructions designed for use as 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 can branch to an instruction in an IT block.
Availability
This 16-bit instruction is available in T32 only.
In A32 code, IT
is a
pseudo-instruction that does not generate any code.
There is no 32-bit version of this instruction.
Correct examples
IT GT LDRGT r0, [r1,#4] IT EQ ADDEQ r0, r1, r2
Incorrect examples
IT NE ADD r0,r0,r1 ; syntax error: no condition code used in IT block ITT EQ MOVEQ r0,r1 ADDEQ r0,r0,#1 ; IT block covering more than one instruction is deprecated IT GT LDRGT r0,label ; LDR (PC-relative) is deprecated in an IT block IT EQ ADDEQ PC,r0 ; ADD is deprecated when Rdn is the PC