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这两天看时间触发嵌入式系统设计模式/pattern for time-triggered edmedded system ,building reliable applications with the 8051 family of microcontrollers，第14章 合作式调度器 这一章，作者写了一个简单的操作系统。
我的感觉如下
1 该系统很简单，没有任务切换，因此不需要保存现场地址
2 每一个任务占用7个自己的RAM
3 看了别人写的这个小操作系统以后，突然感觉操作系统也没有什么神秘的了，他和普通的代码没有任何区别，我们可以把它理解成一般的程序，只不过他是用来调度我们的任务。
作者源码如下
/*------------------------------------------------------------------*-
Main.H (v1.00)
------------------------------------------------------------------
'Project Header' (see Chap 9) for project 0_01_12g (see Chap 14)
COPYRIGHT
---------
This code is from the book:
PATTERNS FOR TIME-TRIGGERED EMBEDDED SYSTEMS by Michael J. Pont
[Pearson Education, 2001; ISBN: 0-201-33138-1].
This code is copyright (c) 2001 by Michael J. Pont.
See book for copyright details and other information.
-*------------------------------------------------------------------*/
#ifndef _MAIN_H
#define _MAIN_H
//------------------------------------------------------------------
// WILL NEED TO EDIT THIS SECTION FOR EVERY PROJECT
//------------------------------------------------------------------
// Must include the appropriate microcontroller header file here
#include <AT89x52.h>
// Include oscillator / chip details here
// (essential if generic delays / timeouts are used)
// -
// Oscillator / resonator frequency (in Hz) e.g. (11059200UL)
#define OSC_FREQ (12000000UL)
// Number of oscillations per instruction (4, 6 or 12)
// 12 - Original 8051 / 8052 and numerous modern versions
// 6 - Various Infineon and Philips devices, etc.
// 4 - Dallas, etc.
//
// Take care with Dallas devices
// - Timers default to *12* osc ticks unless CKCON is modified
// - If using generic code on a Dallas device, use 12 here
#define OSC_PER_INST (12)
//------------------------------------------------------------------
// SHOULD NOT NEED TO EDIT THE SECTIONS BELOW
//------------------------------------------------------------------
typedef unsigned char tByte;
typedef unsigned int tWord;
typedef unsigned long tLong;
// Misc #defines
#ifndef TRUE
#define FALSE 0
#define TRUE (!FALSE)
#endif
#define RETURN_NORMAL (bit) 0
#define RETURN_ERROR (bit) 1
//------------------------------------------------------------------
// Interrupts
// - see Chapter 13.
//------------------------------------------------------------------
// Generic 8051/52 timer interrupts (used in most schedulers)
#define INTERRUPT_Timer_0_Overflow 1
#define INTERRUPT_Timer_1_Overflow 3
#define INTERRUPT_Timer_2_Overflow 5
// Additional interrupts (used in shared-clock schedulers)
#define INTERRUPT_EXTERNAL_0 0
#define INTERRUPT_EXTERNAL_1 2
#define INTERRUPT_UART_Rx_Tx 4
#define INTERRUPT_CAN_c515c 17
//------------------------------------------------------------------
// Error codes
// - see Chapter 14.
//------------------------------------------------------------------
#define ERROR_SCH_TOO_MANY_TASKS (1)
#define ERROR_SCH_CANNOT_DELETE_TASK (2)
#define ERROR_SCH_WAITING_FOR_SLAVE_TO_ACK (3)
#define ERROR_SCH_WAITING_FOR_START_COMMAND_FROM_MASTER (3)
#define ERROR_SCH_ONE_OR_MORE_SLAVES_DID_NOT_START (4)
#define ERROR_SCH_LOST_SLAVE (5)
#define ERROR_SCH_CAN_BUS_ERROR (6)
#define ERROR_I2C_WRITE_BYTE (10)
#define ERROR_I2C_READ_BYTE (11)
#define ERROR_I2C_WRITE_BYTE_AT24C64 (12)
#define ERROR_I2C_READ_BYTE_AT24C64 (13)
#define ERROR_I2C_DS1621 (14)
#define ERROR_USART_TI (21)
#define ERROR_USART_WRITE_CHAR (22)
#define ERROR_SPI_EXCHANGE_BYTES_TIMEOUT (31)
#define ERROR_SPI_X25_TIMEOUT (32)
#define ERROR_SPI_MAX1110_TIMEOUT (33)
#define ERROR_ADC_MAX150_TIMEOUT (44)
#endif
/*------------------------------------------------------------------*-
---- END OF FILE -------------------------------------------------
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