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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 ARMv8.

The conditional instruction (including branches, but excluding the BKPT instruction) must specify the condition in the {cond} part of its syntax.

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 {cond} part of its syntax. The IT block continues from the next instruction. Using a BKPT or HLT instruction inside an IT block is deprecated.

Note

You can use an IT 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 and CBNZ.
  • TBB and TBH.
  • CPS, CPSID and CPSIE.
  • 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 an IT 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 Rm or Rd 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 Rm, Rdn or Rdm is the PC
CMP, CMN When Rm or Rn is the PC
MUL -
ASR, LSL, LSR, ROR -
AND, BIC, EOR, ORR, TST -
BX, BLX When Rm 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
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