Computer Labs: I/O and Interrupts

Transcription

1 Computer Labs: I/O and Interrupts 2 o MIEIC Pedro F. Souto (pfs@fe.up.pt) October 3, 2010

2 I/O Operation I/O devices are the interface between the computer and its environment Most of the time, the processor is not synchronized with its environment I/O operations are asynchronous wrt the processor operation Usually, I/O devices are much slower than the processor The processor must wait for an I/O device to complete its current operation before it requests the I/O device a new one

3 How Does the Processor Know that an I/O op is done? Polling The processor polls the I/O device, i.e. reads a status register, to find out Response time Highly variable depends on what the processor has to do between consecutive polls. Bandwidth May be high, if: the interface bus is fast the I/O device has a high-bandwidth or a large buffer, e.g. a disk and the processor polls the I/O device frequently Interrupts The I/O device notifies the processor, via the interrupt mechanism Response time Usually responsive depends on the time: interrupts are disabled or higher priority interrupts take to be served Bandwidth Medium to low. It depends on the amount of data ready to transfer on each interrupt

4 The PC Interrupt Hardware

5 PC Interrupts and Lines PIC 1 PIC 2 Device Vector IRQ0 Timer 0x08 IRQ1 Keyboard 0x09 IRQ2 PIC2 0x0A IRQ0 Real Time Clock 0x70 IRQ1 Replace IRQ2 0x71 IRQ2 Reserved 0x72 IRQ3 Reserved 0x73 IRQ4 Mouse 0x74 IRQ5 Math coprocessor 0x75 IRQ6 Hard disk 0x76 IRQ7 Reserved 0x77 IRQ3 Serial port COM2 0x0B IRQ4 Serial port COM1 0x0C IRQ5 Reserved/Sound card 0x0D IRQ6 Floppy disk 0x0E IRQ7 Parallel port 0x0F

6 Interrupt Service Routines (ISR) in Assembly ISRs are executed by the HW upon an interrupt ISRs are usually written in assembly Need to perform I/O operations CPU 686 BITS 32 global isr_name isr_name: push.. ; save all registers used... ; ISR instructions mov al, EOI ; signal EOI out PIC1_CMD, al ; to PIC1 pop... ; restore all registers used iretd END

7 How to Share Data Between an ISR and Other Code? Problem ISRs occur asynchronously, they do not take parameters and they do not return values Solution Use global variables CPU 686 BITS 32 Use the keyword volatile so the compiler does not optimize the code, adding a bug an ISR may change its global variables at any time #include <dpmi.h> #include <go32.h> extern _scd ;declared elsewhere global _isr ;can be used elsew. _isr:... mov byte [_scd], iretd END volatile char scd = 0; void isr(void);... // furious busy waiting while( scd!= 0 ) { printf("%2x, scd); }...

8 Setting an ISR in DJGPP #include <dpmi.h> #include <go32.h> void isr(void); // prototype of handler written in assembly _go32_dpmi_seginfo old_vec, new_vec; // for handler addresses // save current handler address _go32_dpmi_get_protected_mode_interrupt_vector(int_vector, &old_vec); // set segment info for new handler new_vec.pm_selector = _go32_my_cs( ); // code selector new_vec.pm_offset = (unsigned long) isr_handler; // address // install handler in the IDT _go32_dpmi_set_protected_mode_interrupt_vector(int_vector, &new_vec);... // program code // IMPORTANT: restore the original handler _go32_dpmi_ set_protected_mode_interrupt_vector(int_vector, &old_vec);

9 ISR in C and DJGPP Problem An ISR is no standard function: instead of a RET you need an IRET Solution DJGPP may generate a wrapper function that calls IRET for us, but we need to free it in the end void isr_c(void) {... // ISR in C } _go32_dpmi_seginfo old_vec, new_vec; // for handler addresses _go32_dpmi_get_protected_mode_interrupt_vector(int_vector, &old_vec); new_vec.pm_selector = _go32_my_cs( ); // code selector new_vec.pm_offset = (unsigned long) isr_c; // address // allocate a wrapper for iret _go32_dpmi_allocate_iret_wrapper(&new_vect); _go32_dpmi_set_protected_mode_interrupt_vector(int_vector, &new_vec);... // program code _go32_dpmi_ set_protected_mode_interrupt_vector(int_vector, &old_vec); // free the wrapper function _go32_dpmi_free_iret_wrapper(&new_vect);

10 Further Reading Using Interrupts DJGPP User s Manual: Interrupts Chapter Overview Interrupt Handlers Tutorial Williams, Allaric, The Dark Art of writing DJGPP Hardware Interrupt Handlers, v1.0