ARM processor specifics

Since architecture version v4 (introduced in ARM7 cores), ARM processors have a new 16-bit instruction set called Thumb (the original 32-bit set is referred to as "ARM"). Since these two sets have different instruction encodings and can be mixed in one segment, we need a way to specify how to disassemble instructions. For this purpose, IDA uses a virtual segment register named 'T'. If its value is 0, then ARM mode is used. Otherwise, Thumb mode is used. ARM is the default mode. Please note that if you change the value of T register for a range, IDA will destroy all instructions in that range because their disassembly is no longer correct.

IDA supports ARM/Thumb Macro Assembler as a target assembler. However, some number formats are not supported by the kernel and will not be accepted by the assembler utility.

To decode Aarch64 (ARM64) instructions the segment must be declared as 64-bit.

Processor options for ARM

Simplify instructions

      If this option is on, IDA will simplify instructions and replace
      them by clearer pseudo-instructions
      For example,
              MOV     PC, LR
      is replaced by
              RET
Disable pointer dereferencing
      If this option is on, IDA will not use =label syntax for
      pointer references.
      For example,
                    LDR     R1, =dst
                    ...
      off_0_1003C   DCD dst
      will be replaced by
                    LDR     R1, off_0_1003C
Enable macros
      If this option is on, IDA will use macro instructions to simplify disassembly
      For example,
              MOV R7,#2
              ADD R7,R7,#3
      will be replaced by
              MOV R7, #5
      Apart from the ADD instruction,
      SUB, ORR, LSL instructions are supported
      Also:
      MVN Rx, #x  => MOV Rx, #~x
No automatic ARM-Thumb switch
      If this option is on, IDA will not propagate
      ARM-Thumb modes automatically when following jumps and calls.
Disable BL jumps detection
      Some ARM compilers in Thumb mode use BL (branch-and-link)
      instead of B (branch) for long jumps, since BL has more range.
      By default, IDA tries to determine if BL is a jump or a call.
      You can override IDA's decision using commands in Edit/Other menu
      (Force BL call/Force BL jump).
      If your target does not use this trick, you can set this option
      and IDA will always treat BL as a call.
Edit ARM architecture options
      This button allows you to edit various features of the ARM architecture.
      This will affect the disassembly of some instructions depending on whether
      the selected architecture supports them. For details, see the ARM Architecture Reference Manual.

Command-line options

      You can configure the architecture options from the command line.
      For that, use the -parm:<option1[;option2...]> switch.
      The following options are accepted:
      ARMv<N>  - base ARM architecture version (e.g. ARMv4, ARMv4T,
                 ARMv5TE, ..., ARMv7-M, ARMv7-A)
       or
      <name>   - ARM core name (e.g. ARM7TDMI, ARM926EJ-S, PXA270,
                Cortex-M3, Cortex-A8)
      Additionally, a special name "armmeta" can be used to enable decoding of all known instructions.
      The options above will set some default values that can be adjusted further:
      NoVFP/VFPv<N>           - disable or enable support for VFP
                                instructions (e.g. VFPv3).
      NoNEON/NEON/NEON-FMA    - disable or enable support for NEON
                                (aka Advanced SIMD) instructions.
      NoThumb/Thumb/Thumb-2   - disable or enable support for Thumb (16-bit)
                                or Thumb-2 (16/32-bit) instructions.
      NoARM/ARM               - disable or enable support for ARM
                                instructions.
      XScale                  - support for XScale-specific instructions.
                                Implies ARMv5TE.
      NoWMMX/WMMXv1/WMMXv2    - support for Intel Wireless MMX
                                extensions (v1 or v2). Implies XScale.
See
      Change segment register value
      Set default segment register value
commands to learn how to specify the segment register value.
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