线程和上下文切换C++

我需要帮助来理解以下代码段。它是用 C++(BC31 编译器(编写的线程工作和上下文切换的示例。

我理解为什么PCB存在是为了切换上下文(特别是为了保持SS和SP寄存器(，我也明白通过使用这个程序可以 回到它被打断的点，就像什么都没发生一样。关于代码，我了解宏DISABLE_INTERRUPT和ENABLE_INTERRUPT的用途 (忽略代码某些敏感部分的中断(。我还了解函数createProcess，其中创建了每个线程的本地堆栈以及 线程的标志、段和偏移量将写入其中。在函数 timerISR 中，我了解正在完成上下文切换的部分(写入和读取 SS 和 SP(。

对于其余的代码，我不能说我理解它。函数 returnNextThread((， initNewRoutine((， returnOldRoutine((， finishThread(( 的意义何在？

不过，我最感兴趣的是这个程序是如何同时运行的，那里实际发生了什么，以便这三个函数在运行时交错......

这一切是如何工作的？ 我真的很感激一个简单的解释。

#include<stdio.h>
#include<stdlib.h>
#include<iostream.h>
#include<dos.h>
typedef struct PCB_struct {
unsigned ss;
unsigned sp;
unsigned finished;
unsigned quant;
} PCB;
#define DISABLE_INTERRUPT  asm cli
#define ENABLE_INTERRUPT  asm sti
PCB *threads[4];
volatile unsigned addressOfInterruptVector = 0x08;
volatile unsigned adressOfFreePlaceForInterrupt = 0x60;
volatile unsigned numberOfInterrupts=0;
volatile PCB *activeThread;
volatile unsigned activeThreadNumber=0;
volatile unsigned numberOfFinishedThreads=0;
volatile int necessarilyContextSwitch=0;
PCB* returnNextThread() {
if(activeThreadNumber==0) {
if(threads[1]->finished==0) {
activeThreadNumber=1;
return threads[1];
}
else if(threads[2]->finished==0) {
activeThreadNumber=2;
return threads[2];
}
else if(threads[3]->finished==0) {
activeThreadNumber=3;
return threads[3];
}
else {
activeThreadNumber=0;
return threads[0];
}
}
else if(activeThreadNumber==1) {
if(threads[2]->finished==0) {
activeThreadNumber=2;
return threads[2];
}
else if(threads[3]->finished==0) {
activeThreadNumber=3;
return threads[3];
}
else {
activeThreadNumber=0;
return threads[0];
}
}
else if(activeThreadNumber==2) {
if(threads[1]->finished==0) {
activeThreadNumber=1;
return threads[1];
}
else if(threads[3]->finished==0) {
activeThreadNumber=3;
return threads[3];
}
else {
activeThreadNumber=0;
return threads[0];
}
}
else if(activeThreadNumber==3) {
if(threads[2]->finished==0) {
activeThreadNumber=2;
return threads[2];
}
else if(threads[1]->finished==0) {
activeThreadNumber=1;
return threads[1];
}
else {
activeThreadNumber=0;
return threads[0];
}
}
activeThreadNumber=0;
return threads[0];
}
unsigned tmpSs=0;
unsigned tmpSp=0;
void interrupt timerISR() {
if(!necessarilyContextSwitch) numberOfInterrupts--;
if(numberOfFinishedThreads<3 && (numberOfInterrupts==0 || necessarilyContextSwitch==1)) {
asm {
mov tmpSs,ss
mov tmpSp,sp
}
activeThread->ss=tmpSs;
activeThread->sp=tmpSp;
activeThread=returnNextThread();
tmpSs=activeThread->ss;
tmpSp=activeThread->sp;
numberOfInterrupts=activeThread->quant;
asm {
mov ss,tmpSs
mov sp,tmpSp
}
}
if(!necessarilyContextSwitch) asm int 60h;
necessarilyContextSwitch=0;
}
unsigned oldRoutineOffset, oldRoutineSegment;
void initNewRoutine() {
unsigned offsetAddress=addressOfInterruptVector*4;
unsigned segmentAddress=addressOfInterruptVector*4+2;
unsigned emptyOffset=adressOfFreePlaceForInterrupt*4;
unsigned emptySegment=adressOfFreePlaceForInterrupt*4+2;
DISABLE_INTERRUPT 
asm {
push es
push ax
push di
mov ax,0
mov es,ax
mov di, word ptr segmentAddress
mov ax, word ptr es:di
mov word ptr oldRoutineSegment, ax
mov word ptr es:di, seg timerISR
mov di, word ptr offsetAddress
mov ax, word ptr es:di
mov word ptr oldRoutineOffset, ax
mov word ptr es:di, offset timerISR
mov di, word ptr emptyOffset
mov ax, word ptr oldRoutineOffset
mov word ptr es:di, ax
mov di, word ptr emptySegment
mov ax, word ptr oldRoutineSegment
mov word ptr es:di, ax
pop di
pop ax
pop es
}
ENABLE_INTERRUPT 
}
void returnOldRoutine() {
unsigned offsetAddress=addressOfInterruptVector*4;
unsigned segmentAddress=addressOfInterruptVector*4+2;
DISABLE_INTERRUPT 
asm {
push es
push ax
push di
mov ax,0
mov es,ax
mov di, word ptr segmentAddress
mov ax, word ptr oldRoutineSegment
mov word ptr es:di, ax
mov di, word ptr offsetAddress
mov ax, word ptr oldRoutineOffset
mov word ptr es:di, ax
pop di
pop ax
pop es
}
ENABLE_INTERRUPT 
}
int finishThread() {
necessarilyContextSwitch=1;
DISABLE_INTERRUPT 
activeThread->finished=1;
cout << "Thread " << activeThreadNumber << " finished." << endl;
ENABLE_INTERRUPT 
timerISR();
return 0;
}
void function1() {
for(int i=0;i<30;i++) {
cout << "Execution: function 1: " << i << endl;
for(int j=0;j<10000;j++) {
for(int k=0;k<30000;k++);
}
}
finishThread();
}
void function2() {
for(int i=0;i<30;i++) {
cout << "Execution: function 2: " << i << endl;
for(int j=0;j<10000;j++) {
for(int k=0;k<30000;k++);
}
}
finishThread();
}
void function3() {
for(int i=0;i<30;i++) {
cout << "Execution: function 3: " << i << endl;
for(int j=0;j<10000;j++) {
for(int k=0;k<30000;k++);
}
}
finishThread();
}
void createProcess(PCB *block, void (*method)()) {
unsigned* st1 = new unsigned[1024];
st1[1023] = 0x200;          
st1[1022] = FP_SEG(method);
st1[1021] = FP_OFF(method);
block->sp = FP_OFF(st1+1012);
block->ss = FP_SEG(st1+1012);
block->finished=0;
}
void mainThread() {
for(int i=0;i<30;i++) {
DISABLE_INTERRUPT 
cout << "Main Thread: " << i << endl;
ENABLE_INTERRUPT 
for(int j=0;j<30000;j++) {
for(int k=0;k<30000;k++);
}
}
}
int main() {
DISABLE_INTERRUPT 
threads[1]=new PCB();
createProcess(threads[1], function1);
threads[1]->quant=20;
threads[2]=new PCB();
createProcess(threads[2], function2);
threads[2]->quant=40;
threads[3]=new PCB();
createProcess(threads[3], function3);
threads[3]->quant=20;
threads[0]=new PCB();
activeThread=threads[0];
activeThreadNumber=0;
activeThread->quant=20;
numberOfInterrupts=activeThread->quant;
ENABLE_INTERRUPT 
initNewRoutine();
mainThread();
returnOldRoutine();
cout << "Main program finished." << endl;
return 0;
}