lolevel_handler_irq: sub   lr, lr, #4              @ correct return address
                      stmia sp, { r0-r12, sp, lr }^ @ store  USR registers
                      mrs   r0, spsr                @ get    USR        CPSR
                      stmdb sp!, { r0, lr }         @ store  USR PC and CPSR

                      mov   r0, sp                  @ set    high-level C function arg. = SP
                      bl    hilevel_handler_irq     @ invoke high-level C function

                      ldmia sp!, { r0, lr }         @ load   USR mode PC and CPSR
                      msr   spsr, r0                @ set    USR mode        CPSR
                      ldmia sp, { r0-r12, sp, lr }^ @ load   USR mode registers
                      movs  pc, lr                  @ return from interrupt