[求助]很难的一段程序，求助几天没人回答的出来，求单片机高手帮忙解释一下啊。

本人正在学，这段程序实在有难度啊，是关于pcf8591A/D转换的，主要是把display函数以前的解释一下就行，越详细越好，谢谢啦！
#include "reg52.h"
#include "pcf8591.h"

void init_t0(void);
void init_pcf8591(void);
void display(unsigned char d);

bit adcflag = 0;
unsigned char dspflag;
unsigned char tflag1;
unsigned char tflag2;
unsigned char code dsp_code_ca[] = {0xc0,0xf9,0xa4,0xb0,0x99,0x92,0x82,0xf8,0x80,0x90};

void main(void)
{
unsigned char adcvalue;

init_pcf8591();
init_t0();
while(1)
{
display(adcvalue);
if(adcflag == 1)
{
adcvalue = adc_pcf8591();
adcflag = 0;
}
}
}

void isr_t0(void) interrupt 1
{
tflag1++;
tflag2++;
if(tflag1 == 16) //4ms
{
tflag1 = 0;
dspflag++;
if(dspflag == 3)
dspflag = 0;
}
if(tflag2 == 80) //20ms
{
tflag2 = 0;
adcflag = 1;
}
}

void init_t0(void)
{
TMOD = 0x02;
TH0 = 0x06;
TL0 = 0x06;
ET0 = 1;
EA = 1;
TR0 = 1;
}

void init_pcf8591(void)
{
iic_start();
iic_sendbyte(0x90);
iic_waitack();
iic_sendbyte(CHANNEL_3);
iic_waitack();
iic_stop();
delay(10);
}

void display(unsigned char d)
{
if((dspflag == 0)&&(d>99))
{
P0 = 0xFF;
P2 |= 0xE0;
P2 &= 0x1F;

P0 = dsp_code_ca[d/100];
P2 |= 0xE0;
P2 &= 0x1F;
P0 = 0x20;
P2 |= 0xC0;
P2 &= 0x3F;
}
if((dspflag == 1)&&(d>9))
{
P0 = 0xFF;
P2 |= 0xE0;
P2 &= 0x1F;

P0 = dsp_code_ca[d%100/10];
P2 |= 0xE0;
P2 &= 0x1F;
P0 = 0x40;
P2 |= 0xC0;
P2 &= 0x3F;
}
if(dspflag == 2)
{
P0 = 0xFF;
P2 |= 0xE0;
P2 &= 0x1F;

P0 = dsp_code_ca[d%100%10];
P2 |= 0xE0;
P2 &= 0x1F;
P0 = 0x80;
P2 |= 0xC0;
P2 &= 0x3F;
}
}