COMP 4550Context Switch

Autonomous Agents Lab, University of Manitobajacky@cs.umanitoba.ca

http://www.cs.umanitoba.ca/~jackyhttp://aalab.cs.umanitoba.ca

Context Switch Outline

1. Save all registers2. Find next task3. Setup thread context4. Return from interrupt

● Area to save the registers○ Reserve memory in task control block (TCB)○ Stack

AVR Interrupt Processing

● When an interrupt occurs, the following events will take place

1. PC is pushed onto stack (2 Bytes)2. ISR is responsible for saving the SREG register3. Save RAMPZ register

○ AtMega128 (>64kB of ROM)

● Sample interrupt handler routinepush __zero_reg__ ; R1push __tmp_reg__ ; R0in __tmp_reg__,_SFR_IO_ADDR(SREG) ; Software has to save SREG/PSWpush __tmp_reg__push r18 ; Save remaining volatile GP registerspush r19...

Interrupt Handler Definition

ISR(CONTEXT_TIMER_ISR, ISR_NAKED ISR_BLOCK) { // save state // general and special purpose registersasm volatile( "\t push __zero_reg__\n" "\t push __tmp_reg__\n" "\t in __tmp_reg__,__SREG__\n" "\t push __tmp_reg__\n"#if defined(RAMPZ) "\t in __tmp_reg__,%0\n" "\t push __tmp_reg__\n"#endif "\t clr __zero_reg__\n" "\t push r2\n" "\t push r3\n" ... : : "I"(_SFR_IO_ADDR(RAMPZ)) : );

AVR Interrupt Processing

To restore context

1. Setup stack pointer to point to common stack2. pop registers, SREG, RAMPZ3. RETI (Enables I Bit in SREG)