如果禁用硬件中断（在第37行注释），则第38〜40行中的代码将继续依次调用过程task0和task1，分别打印字母C和S（全屏打印并从左上角开始到再次打印）。 问题是一旦启用中断（第37行），并注释掉第38〜40行，程序就不会交替从task0切换到task1。它只打印字母S，并且可能会随着时间的推移而闪烁。
代码位于引导扇区中（前512个字节）。问题应该在以下之一： 第20-21行在第8个处理程序位置将处理程序设置为IVT表； 第23-30行设置计时器8293; 第77-92行是计时器中断处理程序。
要运行该程序，请将以下源代码复制到一个文件中，将其命名为ep3.s，然后在Terminal（QEMU仿真器，在Linux上需要运行ld和ld）中键入以下行：
as -o ep3.o ep3.s;ld ep3.o -o ep3 -Ttext=0x7c00 --oformat=binary;sudo qemu-system-x86_64 -drive format=raw,file=ep3 -cpu max
源代码：
.code16 #! tell the assembler to generate 16-bit code
.globl _start # comments start with '!',to be altered at a later stage
.section .text
_start:
cli # Turn off interrupts
xor %ax, %ax # Initialize the segments, set the stack to grow down from
mov %ax, %ds # start of bootloader at _start. SS:SP=0x0000:0x7c00
mov %ax, %ss
mov $_start, %sp
mov %ax, %es
sti # Enable interrupts
cld # Set direction flag forward for string instructions
ljmp *jump_offset
main: # main program starts
#call setup_ivt
cli
movl $system_interrupt, %eax # set IVT int 0x80
movl %eax, 0x0200(,1)
movl $timer_interrupt, %eax # set timer handler
movl %eax, 0x0020(,1) # timer part 1
movb $0x36, %al # 8293 timer setting
movl $0x43, %edx
outb %al, %dx
movl $11930, %eax # about every 1/100 second
movl $0x40, %edx
outb %al, %dx
movb %ah, %al
outb %al, %dx
mov $0xE, %al # 8259A, mask other interrupts except interrput IRQ1
out %al, $0x21
mov $0xF, %al
out %al, $0xA1
sti
#l1: call task0
# call task1
# jmp l1
jmp .
write_char: # set %ax, SCN_SEL,scn_pos before call
pushl %ebx # put %ebx to stack
pushw %es
movw SCN_SEL, %es
movl scn_pos, %ebx
cmpl $2000, %ebx
jb 1f
call clear_screen
xor %ebx, %ebx
1: shll $1, %ebx # time %bx by 2
movw %ax, %es:(%bx)# write 2 bytes to screen, 1 character displayed
shrl $1, %ebx
addl $1, %ebx
movl %ebx, scn_pos
popw %es
popl %ebx # recover %ebx from stack
ret
# end of function
system_interrupt: #system_interrupt handler to call write_char and iret;
push %ds
pushl %edx
pushl %ecx
pushl %ebx
pushl %eax
call write_char
popl %eax
popl %ebx
popl %ecx
popl %edx
pop %ds
iret
#end of system_interrupt
timer_interrupt: #timer part 2
pushl %eax
movb $0x20, %al
outb %al, $0x20
cli
movb $1, %al
cmpb %al, current
je st0
movb %al, current
call task1
jmp st1
st0:movb $0, current
call task0
st1:popl %eax
sti
iret
#ep3 add two functions
task0:
pushl %eax
pushl %ecx
movb $0x0A, %ah # set the colour attributes
movb $67, %al
int $0x80
movl $0xFFFF, %ecx # pause for some time
2: loop 2b
# jmp task0
popl %ecx
popl %eax
ret
task1:
pushl %eax
pushl %ecx
movb $0x0D, %ah # set the colour attributes
movb $83, %al
int $0x80
movl $0xFFFF, %ecx
3: loop 3b
#jmp task1
popl %ecx
popl %eax
ret
#ignore_int:
# pushl %eax
# movb $0x0E, %ah
# movb $84, %al
# # call write_char
# int $0x80
# popl %eax
# iret
#
#setup_ivt:
# pushl %eax
# pushl %edx
# movl $0, %edx
# movl $0, %eax
# lea ignore_int, %eax
#loop_ivt:
# movl %eax, %es:(,%edx,4)
# # movw $0, %es:0x2(,%edx,4)
# addl $1, %edx
# cmpl $256, %edx
# jb loop_ivt
# popl %edx
# popl %eax
# ret
#
clear_screen:
pushl %eax
pushl %ebx
pushl %ecx
pushl %edx
mov $0x0700, %ax # function 07, AL=0 means scroll whole window
mov $0x0007, %bh # character attribute = white on black
mov $0x0000, %cx # row = 0, col = 0
mov $0x184f, %dx # row = 24 (0x18), col = 79 (0x4f)
int $0x10 # call BIOS video interrupt
popl %edx
popl %ecx
popl %ebx
popl %eax
ret
#.section .data
jump_offset : .word main
jump_segment: .word 0x0
SCN_SEL: .word 0xb800 #! memory number where colour text starts
scn_pos: .word 0x0 # 2 bytes to save current screen postion
current: .byte 0x1
.org 510 # to advance the location to 510th byte
.word 0xAA55 # last 16 bits of first 512 bytes on disk
#as -o ep3.o ep3.s;ld ep3.o -o ep3 -Ttext=0x7c00 --oformat=binary;sudo qemu-system-x86_64 -drive format=raw,file=ep3 -cpu max
程序预期交替打印字母C和S。每次计时器处理程序运行时，它都会检查当前任务是否是显示C的task0，然后切换到显示S的任务1，反之亦然。 实际：屏幕上只有一个字母闪烁。