Input/Output Interface

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Presentation transcript:

Input/Output Interface Address bus Data bus Control bus address decoder address decoder fffffffc Addr DE I/O Interface Mem 1 ... Mem n Keyboard

How to do I/O By “Polling” + Very simple and fast li $t1 0xfffffffc ;Interface address waitkey: lb $t0 0($t1) ;Poll Keybard nop beq $t0 $r0 waitkey ;no key strike + Very simple and fast - Can’t do any useful work while waiting - Only OK in a single process system, might miss data if other process takes over

External Interrupt Let the Interface generate an External Interrupt when data is available + Access I/O Interface only when needed + I/O handling is done by OS (Operating System) I/O addresses can be “privileged” for SAFETY + Data is never lost (if processor fast enough) - Slower than “Polling” We will address that problem later

Input/Output Interface Address bus Data bus Control bus CTRL[INT2]=keyboard strike fffffffc Addr DE I/O Interface Moore about: Buffered I/O “Intelligent Devices” etc. later. Keyboard

An External Interrupt Occurs The “context” of the User program must be restored User Program Kernel Program . .. bne $t0 $r0 upp mfc0 $k0 $14 jr $k0 rfe add $t0 $t1 $t2 .. .

Multiple Processes User 1 User 2 User 3 Kernel .text .text .text .ktext Instructions .data .data .data .kdata Data

Time Slicing, Context Switch On Each Timer Interrupt Store the User program “Context” Choose next User program (process) Round robin process scheduling Restore its “Context” Pass control to User program

Process Control Block (PCB) Resume Address (next PC for this process) Global Data pointer ($gp) Stack pointer ($sp) All registers but $k0, $k1, $gp, $sp $k0, $k1 might be trashed by the Kernel code $gp, $sp stored in the PCB

Process Control Block PCB for Process p1 next_pc $gp $sp .text resume addr base for global data area for p1 .data top of user p1 stack .p1 stack $pc,$sp,$gp are initiated by the OS at “run” time

Where are the 28 registers? PCB p1 PCB p1 next_pc $gp $sp Let’s put them on the user program’s stack! next_pc $gp $sp And adjust the $sp $t0 $t1 … .. $t0 $t1 … .. .p1 stack .p1 stack

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