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        .globl _start

        #

        # NOTE: Registers r10-r11 are reserved for the interrupt handler

        #       while the others can be used by the main loop/start code.

_start:

        # patch the external interrupt handlers entry

        add r1, r0, handler

        ldw r2, @(r1, 0)

        ldw r3, @(r1, 4)

        add r1, r0, 0xfffff138

        stw r2, @(r1, 0)

        stw r3, @(r1, 4)

        # enable external interrupts - cr0 == PSW

        mvfsys r2, cr0

        or r2, r0, 0x04000000

        mvtsys cr0, r2

        # wait for flag to be set

loop:

        add r2, r0, flag

        ldw r3, @(r2, 0)

        bratzr r3, loop

        # clear the flag

        stw r0, @(r2, 0)

        add r2, r0, tick

        # putstr

        .long 0x0e000001, 0x00f00000

        bra loop

        # finished

        add r2, r0, r0 || nop

        .long 0x0e000004, 0x00f00000

handler:

        jmp real_handler

real_handler:

        add r10, r0, 1

        add r11, r0, flag

        stb r10, @(r11,0)

        reit

flag:   .long 0

tick:   .ascii "Tick\r\n"