computer organization lecture 21

Fall 2006

Lillevik 333f06-l21 1University of Portland School of Engineering

EE 333

Computer OrganizationLecture 21

Subroutines, stackInterrupts, service routines

Input and output, buses

Fall 2006


EE 333

Instruction flow

• Most of the time: program executes sequentially unless a branch or jump requested

• Sometimes– Programmer wishes to execute a subroutine,

function, or method– An unusual event occurs internally or

externally and the program needs to respond to it

Fall 2006


EE 333

Subroutine flow control

1. Program prior to sub call2. Caller transfers control to Callee3. Callee transfers control to Caller4. Program after sub call

Caller Callee

12

34

May be nested

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EE 333

Hardware for subroutines

• Goal– Jump to some memory location– Return back to next instruction after jump (PC-

next)

• Requirements– Save the value of the PC-next– Modify the PC, do the jump– Restore the value of PC to PC-next

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EE 333

MIPS subroutines

• Jump-and-link: jal– Saves PC-next in $ra– Jumps to subroutine address

• Jump register: jr– Jumps to address in register– jr $ra will perform return

• Use these to access subroutines

Fall 2006


EE 333

Subroutine programming

CallCall-next

subreturn

CallCall-next

PC = Call

$ra = xx

PC = sub

$ra = Call-next

PC = Call-next

$ra = Call-next

jal sub

jr $ra

memory memory memory

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EE 333

How do we nest subroutines?

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Stack memory usage

Push onto stack Pop from stack

$sp points to top of stack

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Stack operations

• Pushsub $sp, $sp, 4 # push $ra on stack

sw $ra, ($sp)

• Poplw $ra, ($sp) # pop $ra from stack

add $sp, $sp, 4

Same operations to save registers

Fall 2006


EE 333

Unusual events

• Three types: all require immediate action

– Internal to computer• Hardware generated

• Software generated (OS requests)

– External to computer: only hardware generated

• Notation– MIPS: exception = internal, interrupt = external

– Common: everything is an interrupt

Fall 2006


EE 333

Identify possible events?

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Hardware for interrupts

• Enable/disable: turns interrupts on and off

• Controller changes and response– Finish instructions (flush pipeline)– Disable interrupts– Record type of event (Cause register)– Save return address (EPC register)– Branch (vector) to specific address (0x8000 0180)

• Mechanism nearly identical to jal

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EE 333

MIPS controller state diagram• Two sources: bad

opcode, overflow

• Cause register written

• EPC written

• PC written

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EE 333

Interrupt request map

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EE 333

Interrupt service routine

• Save any registers used: save state on stack

• Perform required action• Restore registers used: restore state from stack

• Enable interrupts– May be done after registers saved– Provides for nested interrupts

• Return

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EE 333

Major computer components

Five classic computer components

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EE 333

Computer components

• Input: receives information from external world

• Output: transmits information to external world

• Memory: holds programs and data

• Data path: physical route that carries info

• Control: coordinates overall flow of info

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System bus

Bus interconnects system agents

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MDP16 busesAdr, Data,

Control

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Early PC busesHigh speed

Med speed

Slow speed

CPU’s

8080 – 80486

vintage

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Pentium 4 buses

82850

82801

300/400 MHzRDRAM DIMMS

6 Slots

100 Mbit

North bridge

South bridge

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System view of a computer

One Agent at-a-time owns the bus

···

Signal 0

Signal n

Agent 0 Agent n···

BusSignal 1

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Example buses?

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Bus properties

• Agents– Master: owns bus, issues requests– Slave: responds to requests from a master

• Interconnections– Parallel, serial– Point-to-point, distributed– Single- and multi-master

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Bus properties, continued.

• Electrical– Clocking: synchronous, asynchronous– Logic families: setup, hold, propagation– Physical: impedance, length, speed

• Performance– Bandwidth or data rate, B/s– Turn-around time to change masters, t

Fall 2006


EE 333

Find B/W of MDP16 memory?

CLK = 200 ns, Width = 16-bits

Fall 2006


EE 333

Fall 2006


EE 333

How do we nest subroutines?

• Use stack pointer register ($sp)

• Push $ra onto stack upon subroutine entry

• Execute body of subroutine, may call other subroutines

• Pop $ra from stack just before subroutine exit

• Return from subroutine

Fall 2006


EE 333

Identify possible events?

• Overflow, underflow, divide-by-zero, parity or CRC error, illegal instruction

• Memory or file protection (write to read-only, page fault)

• Real time clock, OS request

• Disk drive, CD, E-net: data transfer ready, or complete

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Example buses?

• Uart

• I/O

• Address

• PCI

• ISA

• S-ATA

• USB

http://en.wikipedia.org/wiki/Computer_bus

Fall 2006


EE 333

Find B/W of MDP16 memory?

CLK = 200 ns, Width = 16-bits

snsclkTlw

0.120055

sMBs

bytes

s

bitsBW 0.2

10

2

0.1

166

computer organization lecture 21

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