The current ASM interrupt handler for pre ARMv7 architectures looks like this:
_asm_default_irq_handler:
sub lr, lr, 4 // make sure we jump back to the right place
push {{ lr }} // save adjusted LR to IRQ stack
mrs lr, spsr // The hardware has copied the interrupted task's CPSR to SPSR_irq - grab it and
push {{ lr }} // save it to IRQ stack using LR
msr cpsr_c, {sys_mode} // switch to system mode so we can handle another interrupt (because if we interrupt irq mode we trash our own shadow registers)
mov lr, sp // align SP down to eight byte boundary using LR
and lr, lr, 7 //
sub sp, lr // SP now aligned - only push 64-bit values from here
push {{ r0-r3, r12, lr }} // push alignment amount (in LR) and preserved registers
"#,
crate::save_fpu_context!(),
r#"
bl _irq_handler // call C handler (they may choose to re-enable interrupts)
"#,
crate::restore_fpu_context!(),
r#"
pop {{ r0-r3, r12, lr }} // restore alignment amount (in LR) and preserved registers
add sp, lr // restore SP alignment using LR
msr cpsr_c, {irq_mode} // switch back to IRQ mode (with IRQ masked)
pop {{ lr }} // load and restore SPSR using LR
msr spsr, lr //
ldmfd sp!, {{ pc }}^ // return from exception (^ => restore SPSR to CPSR)
.size _asm_default_irq_handler, . - _asm_default_irq_handler
"#,Is this not smashing the banked LR of system mode? The ARMv7M + interrupt handler actually stores the LR by pushing it before using it for stack alignment. I tried to do the same for the interrupt handler above and the tests are still passing. I am just wondering why the tests are working in the first place. Was the LR not saved deliberately and I am just missing something?
The current ASM interrupt handler for pre ARMv7 architectures looks like this:
Is this not smashing the banked LR of system mode? The ARMv7M + interrupt handler actually stores the LR by pushing it before using it for stack alignment. I tried to do the same for the interrupt handler above and the tests are still passing. I am just wondering why the tests are working in the first place. Was the LR not saved deliberately and I am just missing something?