小弟有一个个程序看不懂，望各位大哥，大姐们帮帮忙，帮我解释一下，小弟不甚感激！
#define TRUE 1
#define FALSE 0
#define INVALID -1
#define NULL 0
#define total_instruction 320
#define total_vp 32
#define clear_period 50
typedef struct{
    int pn,pfn,counter,time;
}p1_type;
p1_type p1[total_vp];
struct pfc_struct{
    int pn,pfn;
    struct pfc_struct *next;
};
typedef struct pfc_struct pfc_type;
pfc_type pfc[total_vp],*freepf_head,*busypf_head,*busypf_tail;
int diseffect,  a[total_instruction];
int page[total_instruction],  offset[total_instruction];
void initialize();
void FIFO();
//void LRU();
void main()
{
    int S,i,j;
    srand(getpid()*10);
    S=((float)319*rand())/32767/32767/2+1;
    for(i=0;i<total_instruction;i+=4)
    {
        a[i]=S;
        a[i+1]=a[i]+1;
        a[i+2]=((float)a[i]*rand())/32767/32767/2;
        a[i+3]=a[i+2]+1;
        S=(float)rand()*(318-a[i+2])/32767/32767/2+a[i+2]+2;
    }
    for(i=0;i<total_instruction;i++)
    {
        page[i]=a[i]/10;
        offset[i]=a[i]%10;
        
    }
    for(i=4;i<=32;i++)
    {
        printf("%2d page frames   ",i);
                FIFO(i);
        printf("\n");
    }
    
}

void FIFO(total_pf)
    int total_pf;
{
    int i,j;
    pfc_type *p,*t;
    initialize(total_pf);
    busypf_head=busypf_tail=NULL;
    for(i=0;i<total_instruction;i++)
    {
        if (p1[page[i]].pfn==INVALID)
        {
            diseffect+=1;
            if(freepf_head==NULL)
            {
                p=busypf_head->next;
                p1[busypf_head->pn].pfn=INVALID;
                freepf_head=busypf_head;
                freepf_head->next=NULL;
                busypf_head=p;
            }
            p=freepf_head->next;
            freepf_head->next=NULL;
            freepf_head->pn=page[i];
            p1[page[i]].pfn=freepf_head->pfn;
            if(busypf_tail==NULL)
                busypf_head=busypf_tail=freepf_head;
            else
            {
                                busypf_tail->next=freepf_head;
                                busypf_tail=freepf_head;
                
            }
            freepf_head=p;
            }
    }
    printf("    FIFO:%6.4f",1-(float)diseffect/320);
}
void initialize(total_pf)
int total_pf;
{
  int i;
  diseffect=0;
  for(i=0;i<total_vp;i++)
   {
    p1[i].pn=i;
    p1[i].pfn=INVALID;
    p1[i].counter=0;
    p1[i].time=-1;
   }
   for(i=1;i<total_pf;i++)
   {
    pfc[i-1].next=&pfc[i];
    pfc[i-1].pfn=i-1;
    }
   pfc[total_pf-1].next=NULL;
   pfc[total_pf-1].pfn=total_pf-1;
   freepf_head=&pfc[0];
}