本文主要是介绍freeNOS代码走读杂记，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
最近在看github上一个微os的代码，freeNOS.走读的过程中顺便记一下学习的点
枚举用位偏移定义来提高效率
/*** Memory access flags.*/
typedef enum Access
{None = 0,Readable = 1 << 0,Writable = 1 << 1,Executable = 1 << 2,User = 1 << 3,Uncached = 1 << 4,InnerCached = 1 << 5,OuterCached = 1 << 6,Device = 1 << 7
}读，写是通过port口地址加offset值来定位具体内存地址，再进行具体的内存操作
为了提高效率所有的IO操作函数全部都是inline
//IntelIO.h
inline u8 inb(u16 port)
{u8 b;port += m_base;asm volatile ("inb %%dx, %%al" : "=a" (b) : "d" (port));return b;
}/*** Read a word from a port.* @param port The I/O port to read from.* @return Word read from the port.*/
inline u16 inw(u16 port)
{u16 w;port += m_base;asm volatile ("inw %%dx, %%ax" : "=a" (w) : "d" (port));return w;
}/*** Output a byte to a port.* @param port Port to write to.* @param byte The byte to output.*/
inline void outb(u16 port, u8 byte)
{port += m_base;asm volatile ("outb %%al,%%dx"::"a" (byte),"d" (port));
}/*** Output a word to a port.* @param port Port to write to.* @param byte The word to output.*/
inline void outw(u16 port, u16 word)
{port += m_base;asm volatile ("outw %%ax,%%dx"::"a" (word),"d" (port));
}/*** Output a long to a I/O port.* @param port Target I/O port.* @param l The long 32-bit number to output.*/
inline void outl(u16 port, u32 l)
{port += m_base;asm volatile ("outl %%eax,%%dx"::"a" (l),"d" (port));
}/*** Read memory mapped register.** @param addr Address of the register to read.* @return 32-bit value of the register.*/
inline u32 read(Address addr)
{addr += m_base;return *(volatile u32 *) addr;
}/*** Read a number of 32-bit values.** @param addr Address of the starting 32-bit value.* @param count Number of bytes to read.* @param buf Output buffer.*/
inline void read(Address addr, Size count, void *buf)
{for (Size i = 0; i < count; i+= sizeof(u32)){*(u32 *)(((u8 *)buf) + i) = read(addr + i);}
}/*** Write memory mapped register.** @param addr Address of the register to write.* @param data 32-bit value to write in the register.*/
inline void write(Address addr, u32 data)
{addr += m_base;*(volatile u32 *) addr = data;
}寄存器对应bit置1 set()和置0 unset()：
/*** Set bits in memory mapped register.** @param addr Address of the register to write.* @param data 32-bit value containing the bits to set (bitwise or).*/
inline void set(Address addr, u32 data)
{u32 current = read(addr);current |= data;write(addr, current);
}/*** Unset bits in memory mapped register.** @param addr Address of the register to write.* @param data 32-bit value containing the bits to set (bitwise or).*/
inline void unset(Address addr, u32 data)
{u32 current = read(addr);current &= ~(data);write(addr, current);
}地址等关键变量类型通过typedef来定义，提高可读性和健壮性。大量采用了switch-case操作，基本没有if判断操作，通过state去串联（状态机？）。
函数返回值只返回当前操作的状态，其他逻辑信息全部用出参表示。
函数内基本没有任何合法性判断和异常处理流程代码。
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