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Shared Debug.Softwarestep Pseudocode

Library pseudocode for shared/debug/softwarestep/CheckSoftwareStep

// CheckSoftwareStep()
// ===================
// Take a Software Step exception if in the active-pending state

CheckSoftwareStep()

    // Other self-hosted debug functions will call AArch32.GenerateDebugExceptions() if called from
    // AArch32 state. However, because Software Step is only active when the debug target Exception
    // level is using AArch64, CheckSoftwareStep only calls AArch64.GenerateDebugExceptions().
    step_enabled = !ELUsingAArch32(DebugTarget()) && AArch64.GenerateDebugExceptions() && MDSCR_EL1.SS == '1';
    if step_enabled && PSTATE.SS == '0' then
        AArch64.SoftwareStepException();

Library pseudocode for shared/debug/softwarestep/DebugExceptionReturnSS

// DebugExceptionReturnSS()
// ========================
// Returns value to write to PSTATE.SS on an exception return or Debug state exit.

bit DebugExceptionReturnSS(bits(N) spsr)
    if UsingAArch32() then
        assert N == 32;
    else
        assert N == 64;

    assert Halted() || Restarting() ||  PSTATE.EL != EL0;

    if Restarting() then
        enabled_at_source = FALSE;
    elsif UsingAArch32() then
        enabled_at_source = AArch32.GenerateDebugExceptions();
    else
        enabled_at_source = AArch64.GenerateDebugExceptions();

    if IllegalExceptionReturn(spsr) then
        dest = PSTATE.EL;
    else
        (valid, dest) = ELFromSPSR(spsr);  assert valid;

    dest_is_secure = IsSecureBelowEL3() || dest == EL3;
    dest_using_32 = (if dest == EL0 then spsr<4> == '1' else ELUsingAArch32(dest));
    if dest_using_32 then
        enabled_at_dest = AArch32.GenerateDebugExceptionsFrom(dest, dest_is_secure);
    else
        mask = spsr<9>;
        enabled_at_dest = AArch64.GenerateDebugExceptionsFrom(dest, dest_is_secure, mask);

    ELd = DebugTargetFrom(dest_is_secure);
    if !ELUsingAArch32(ELd) && MDSCR_EL1.SS == '1' && !enabled_at_source && enabled_at_dest then
        SS_bit = spsr<21>;
    else
        SS_bit = '0';

    return SS_bit;

Library pseudocode for shared/debug/softwarestep/SSAdvance

// SSAdvance()
// ===========
// Advance the Software Step state machine.

SSAdvance()

    // A simpler implementation of this function just clears PSTATE.SS to zero regardless of the
    // current Software Step state machine. However, this check is made to illustrate that the
    // processor only needs to consider advancing the state machine from the active-not-pending
    // state.
    target = DebugTarget();
    step_enabled = !ELUsingAArch32(target) && MDSCR_EL1.SS == '1';
    active_not_pending = step_enabled && PSTATE.SS == '1';

    if active_not_pending then PSTATE.SS = '0';

    return;

Library pseudocode for shared/debug/softwarestep/SoftwareStep_DidNotStep

// Returns TRUE if the previously executed instruction was executed in the inactive state, that is,
// if it was not itself stepped.
// Might return TRUE or FALSE if the previously executed instruction was an ISB or ERET executed
// in the active-not-pending state, or if another exception was taken before the Software Step exception.
// Returns FALSE otherwise, indicating that the previously executed instruction was executed in the
// active-not-pending state, that is, the instruction was stepped.
boolean SoftwareStep_DidNotStep();

Library pseudocode for shared/debug/softwarestep/SoftwareStep_SteppedEX

// Returns a value that describes the previously executed instruction. The result is valid only if
// SoftwareStep_DidNotStep() returns FALSE.
// Might return TRUE or FALSE if the instruction was an AArch32 LDREX or LDAEX that failed its condition code test.
// Otherwise returns TRUE if the instruction was a Load-Exclusive class instruction, and FALSE if the
// instruction was not a Load-Exclusive class instruction.
boolean SoftwareStep_SteppedEX();