51 MCU interrupt function

Interrupts are implemented using the interrupt keyword and terminal numbers 0-4:
Return value Function name  interrupt n
n corresponds to the number of the interrupt source. The interrupt number tells the interrupter the entry address of the interrupt program, which corresponds to the enable bit in the IE register, that is, bit 0 in the IE register corresponds to external interrupt 0. The
interrupt sources and terminal numbers of the 8051 microcontroller are as follows:
 

Interrupt number	Interrupt Sources	Entry address
0	External interrupt 0	0003H
1	Timer/Counter 0 Overflow	000BH
2	External interrupt 1	0013H
3	Timer/Counter 1 Overflow	001BH
4	Serial port interrupt	0023H

 
In the 51 series of single-chip microcomputers, some single-chip microcomputers have up to 32 interrupt sources, so the terminal numbers are 0-31.
When a specific task is being executed, there may be more urgent things that need to be handled by the CPU, which involves interrupt priority. High-priority interrupts can interrupt low-priority interrupt programs that are being processed, so it is best to assign different register groups to each priority program. In C51, you can use using to specify the register group. The variable after using is a constant integer from 0 to 3, which represents the 4 register groups in the 51 single-chip microcomputer. The complete syntax of the interrupt function is as follows:
 return value  function name ([parameter]) [mode] [reentry] interrupt n [usingn]
unsigned int interruptcnt;
unsigned char second;
void timer0 (void) interrupt 1 using 2
if (++interruptcnt==4000)   // count to 4000
second++;         // another counter
interruptcnt=0;   // counter clear