8255A设置为C口输入，A口输出。C口和报警器的按键K0-K7相连，读取C口数据，只要有一个键按下，则指示灯亮，且喇叭响，喇叭发声的频率由8253的输出端OUT1控制。 汇编语言源代码

Assuming that the 8255A is connected to port 0 of the microcontroller, and 8253 is connected to port 1, the following is the assembly code:

ORG 0H
MOV A, #0FFH   ; Set all bits of port 0 as input
OUT 0H, A
MOV A, #00H    ; Set all bits of port 1 as output
OUT 1H, A

LOOP:
IN A, 0H       ; Read the input from port 0
ANL A, #0FH    ; Mask out the upper 4 bits
JZ NOALARM     ; Jump if no key is pressed
SETB P1.0      ; Turn on the LED
MOV A, #0F0H   ; Set the lower 4 bits of port 1 to 1111b
OUT 1H, A
CALL DELAY     ; Call delay subroutine
CLR P1.0       ; Turn off the LED
MOV A, #00H    ; Set the lower 4 bits of port 1 to 0000b
OUT 1H, A
CALL DELAY     ; Call delay subroutine
SJMP LOOP      ; Jump back to the loop

NOALARM:
CLR P1.0       ; Turn off the LED
MOV A, #00H    ; Set the lower 4 bits of port 1 to 0000b
OUT 1H, A
SJMP LOOP      ; Jump back to the loop

DELAY:
MOV R1, #20H   ; Load R1 with a value for delay
AGAIN1:
MOV R2, #20H   ; Load R2 with a value for delay
AGAIN2:
DJNZ R2, AGAIN2
DJNZ R1, AGAIN1
RET           ; Return from delay subroutine

In this code, the input from port 0 is read and the upper 4 bits are masked out. If any of the lower 4 bits are set, the LED is turned on and the alarm is sounded using the output from port 1, which is controlled by the 8253. The frequency of the alarm is determined by the output from OUT1 of the 8253. A delay subroutine is called to create a delay between turning the LED and alarm on/off to create a flashing effect.