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SMC instruction

Dm Sh
Note: This was originally posted on 22nd January 2013 at http://forums.arm.com

Good day.

I have a question - where I can get the #immediate value (4bit) in Secure Monitor Exception Handler, when called SMC instruction with non zero parametr - #imm-4 ?
Don't found this value in any registers.


From ARM documentation.
-------------------------------------

SMC
Secure Monitor Call.


Syntax
SMC{cond} #imm4 where:

[i]imm4[/i]is a 4-bit immediate value. This is ignored by the ARM processor, but can be used by the SMC exception handler to determine what service is being requested.


Thanks.
Parents
  • Arm Employee Badge
    Note: This was originally posted on 27th January 2013 at http://forums.arm.com

    Bear in mind that the SMC instruction is used to switch worlds, so you tend to get an address in the "other world's" virtual address map in LR, which in many cases may not be the same as the currently running software. Direct access of the address in LR is therefore unlikely to do what you want - it may well page fault, or at least return utterly unrelated data.

    This generally makes it very hard to use this feature of the SMC instruction; at least it makes it more hassle than it is worth - in most cases putting a constant in a register by hand as part of the cross-world smc call API is much easier.

    HTH,
    Iso
Reply
  • Arm Employee Badge
    Note: This was originally posted on 27th January 2013 at http://forums.arm.com

    Bear in mind that the SMC instruction is used to switch worlds, so you tend to get an address in the "other world's" virtual address map in LR, which in many cases may not be the same as the currently running software. Direct access of the address in LR is therefore unlikely to do what you want - it may well page fault, or at least return utterly unrelated data.

    This generally makes it very hard to use this feature of the SMC instruction; at least it makes it more hassle than it is worth - in most cases putting a constant in a register by hand as part of the cross-world smc call API is much easier.

    HTH,
    Iso
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