有关此串行通信代码的问题?[CCortex-M4]

Question

我正在查看STMicroelectronics关于使用中断实现USART通信的以下代码

#include <stm32f10x_lib.h>                        // STM32F10x Library Definitions
#include <stdio.h>
#include "STM32_Init.h"                           // STM32 Initialization

/*----------------------------------------------------------------------------
Notes:
The length of the receive and transmit buffers must be a power of 2.
Each buffer has a next_in and a next_out index.
If next_in = next_out, the buffer is empty.
(next_in - next_out) % buffer_size = the number of characters in the buffer.
*----------------------------------------------------------------------------*/
#define TBUF_SIZE   256      /*** Must be a power of 2 (2,4,8,16,32,64,128,256,512,...) ***/
#define RBUF_SIZE   256      /*** Must be a power of 2 (2,4,8,16,32,64,128,256,512,...) ***/
/*----------------------------------------------------------------------------
*----------------------------------------------------------------------------*/
#if TBUF_SIZE < 2
#error TBUF_SIZE is too small.  It must be larger than 1.
#elif ((TBUF_SIZE & (TBUF_SIZE-1)) != 0)
#error TBUF_SIZE must be a power of 2.
#endif
#if RBUF_SIZE < 2
#error RBUF_SIZE is too small.  It must be larger than 1.
#elif ((RBUF_SIZE & (RBUF_SIZE-1)) != 0)
#error RBUF_SIZE must be a power of 2.
#endif
/*----------------------------------------------------------------------------
*----------------------------------------------------------------------------*/
struct buf_st {
unsigned int in;                                // Next In Index
unsigned int out;                               // Next Out Index
char buf [RBUF_SIZE];                           // Buffer
};
static struct buf_st rbuf = { 0, 0, };
#define SIO_RBUFLEN ((unsigned short)(rbuf.in - rbuf.out))
static struct buf_st tbuf = { 0, 0, };
#define SIO_TBUFLEN ((unsigned short)(tbuf.in - tbuf.out))
static unsigned int tx_restart = 1;               // NZ if TX restart is required
/*----------------------------------------------------------------------------
USART1_IRQHandler
Handles USART1 global interrupt request.
*----------------------------------------------------------------------------*/
void USART1_IRQHandler (void) {
volatile unsigned int IIR;
struct buf_st *p;
IIR = USART1->SR;
if (IIR & USART_FLAG_RXNE) {                  // read interrupt
USART1->SR &= ~USART_FLAG_RXNE;             // clear interrupt
p = &rbuf;
if (((p->in - p->out) & ~(RBUF_SIZE-1)) == 0) {
p->buf [p->in & (RBUF_SIZE-1)] = (USART1->DR & 0x1FF);
p->in++;
}
}
if (IIR & USART_FLAG_TXE) {
USART1->SR &= ~USART_FLAG_TXE;              // clear interrupt
p = &tbuf;
if (p->in != p->out) {
USART1->DR = (p->buf [p->out & (TBUF_SIZE-1)] & 0x1FF);
p->out++;
tx_restart = 0;
}
else {
tx_restart = 1;
USART1->CR1 &= ~USART_FLAG_TXE;           // disable TX interrupt if nothing to send
}
}
}
/*------------------------------------------------------------------------------
buffer_Init
initialize the buffers
*------------------------------------------------------------------------------*/
void buffer_Init (void) {
tbuf.in = 0;                                    // Clear com buffer indexes
tbuf.out = 0;
tx_restart = 1;
rbuf.in = 0;
rbuf.out = 0;
}
/*------------------------------------------------------------------------------
SenChar
transmit a character
*------------------------------------------------------------------------------*/
int SendChar (int c) {
struct buf_st *p = &tbuf;
// If the buffer is full, return an error value
if (SIO_TBUFLEN >= TBUF_SIZE)
return (-1);
p->buf [p->in & (TBUF_SIZE - 1)] = c;           // Add data to the transmit buffer.
p->in++;
if (tx_restart) {                               // If transmit interrupt is disabled, enable it
tx_restart = 0;
USART1->CR1 |= USART_FLAG_TXE;                // enable TX interrupt
}
return (0);
}
/*------------------------------------------------------------------------------
GetKey
receive a character
*------------------------------------------------------------------------------*/
int GetKey (void) {
struct buf_st *p = &rbuf;
if (SIO_RBUFLEN == 0)
return (-1);
return (p->buf [(p->out++) & (RBUF_SIZE - 1)]);
}

/*----------------------------------------------------------------------------
MAIN function
*----------------------------------------------------------------------------*/
int main (void) {
buffer_Init();                                  // init RX / TX buffers
stm32_Init ();                                  // STM32 setup
printf ("Interrupt driven Serial I/O Examplernrn");
while (1) {                                     // Loop forever
unsigned char c;
printf ("Press a key. ");
c = getchar ();
printf ("rn");
printf ("You pressed '%c'.rnrn", c);
} // end while
} // end main

我的问题如下：

1. 在处理程序函数中，语句((p->in - p->out) & ~(RBUF_SIZE-1))何时计算为零以外的值？如果RBUF_SIZE是2的幂，则~(RBUF_SIZE-1)应始终为零。是否检查p->in>p->out？即使这不是真的，条件应该为零，对吧
2. 在下面的行中，将生成语句p->buf [p->in & (RBUF_SIZE-1)] = (USART1->DR & 0x1FF);。为什么代码将p->in与RBUF_SIZE-1进行AND运算
3. 我们在这段代码中使用了什么样的缓冲区？FIFO

Answer 1

1. 不是这样。例如，假设32位算术，如果是RBUF_SIZE == 0x00000100，则是RBUF_SIZE-1 == 0x000000FF和~(RBUF_SIZE-1) == 0xFFFFFF00(这是按位Not，而不是逻辑Not(。因此，您所指的检查实际上与(p->in - p->out) < RBUF_SIZE相同，但尚不清楚为什么它更优越。ARM GCC 7.2.1为两者生成相同长度的代码(-O1(。

2. 当CCD_ 16为无符号时，CCD_ 14与CCD_。同样，当后者更清晰时，不确定为什么会使用前者；当然，它有效地迫使编译器使用AND运算来计算模，但考虑到RBUF_SIZE在编译时已知为二次幂，我猜测大多数编译器都能解决这个问题(同样，ARM GCC 7.2.1当然可以，我刚刚尝试过——无论哪种方式，它都会产生相同的指令(。

3. 看起来像。FIFO实现为循环缓冲区。