某 8086 系统中，采用单片 8259A 进行中断管理。假设 8259A 工作在普通全嵌套方式，采用普通 EOI 命令结束中断、电平沿触发方式请求中断，IR0 对应的中断类型号为88H。8259A 占用的端口地址是 300H 和 301H。请编写 8259A 的初始化程序段。（包括设置中断向量）

以下是8259A的初始化程序段：

; 8259A initialization code
; assume 8259A is at port 300h

; start initialization sequence
mov al, 0x11      ; initialize 8259A
out 0x20, al      ; write to master command register
jmp short init2   ; jump to init2

; initialize master and slave
init2:
mov al, 0x20      ; set master base IRQ to 20h
out 0x21, al      ; write to master data register
mov al, 0x04      ; tell slave about master IRQ2
out 0x21, al      ; write to master data register
mov al, 0x01      ; set 8259A to 8086 mode
out 0x21, al      ; write to master data register

mov al, 0x11      ; initialize 8259A
out 0xA0, al      ; write to slave command register
jmp short init4   ; jump to init4

init4:
mov al, 0x28      ; set slave base IRQ to 28h
out 0xA1, al      ; write to slave data register
mov al, 0x02      ; tell slave about master IRQ2
out 0xA1, al      ; write to slave data register
mov al, 0x01      ; set 8259A to 8086 mode
out 0xA1, al      ; write to slave data register

mov al, 0xFC      ; mask all interrupts except IRQ0
out 0x21, al      ; write to master data register
mov al, 0xFF      ; mask all interrupts on slave
out 0xA1, al      ; write to slave data register

; set interrupt vector for IRQ0
mov al, 0x88      ; IRQ0 interrupt type is 88h
mov ah, 0          ; clear upper 8 bits of AX
mov bl, 0x20       ; IRQ0 is mapped to INT 20h
mov bh, 0          ; clear upper 8 bits of BX
mov [0:0x20], bx  ; set interrupt vector for IRQ0

; enable interrupts
sti

ret

上述程序段中，首先向主片发送0x11，表示初始化8259A。然后设置主片和从片的基本中断号分别为0x20和0x28，告诉从片主片的IRQ2是从片的中断。接着设置8259A为8086模式，并关闭所有中断。最后设置IRQ0的中断向量，并打开中断。