CPE555A: Real-Time Embedded Systems

Spring 2016, arz 1

CPE555A:Real-Time Embedded Systems

Lecture 4Ali Zaringhalam

Stevens Institute of Technology

CS555A – Real-Time Embedded Systems Stevens Institute of Technology

Spring 2016, arz

CS555A – Real-Time Embedded SystemsStevens Institute of Technology 2

Outline

Procedure Calls I/O Exception Handling

Memory Use

Programs use memory to Store executable code Data used by programs during execution

Memory for data must be available Allocated statically at compile time Allocated dynamically at run time

On the program stack for procedure calls In the heap as needed by program

Spring 2016, arz


Spring 2016, arz4 4

Memory Usage

Global variable. Allocated at compile time.

Local/automatic variables. Allocated on the stack at run time.

Dynamic variables. Allocated in the heap area at run time.

CS555A – Real-Time Embedded SystemsStevens Institute of Technology

Spring 2016, arz5CS555A – Real-Time Embedded Systems

Stevens Institute of Technology 5

Procedure Call: Return

opcode=3 immediate

6 26

J-Type Format

Af ter the procedure fi nishes execution it must return to the instruction at the return address. The address of this instruction is stored in $ra (register 31). Thus to return to this address the procedure executes:

jr $ra

so that no new instruction f or the return instruction is required. Similar to the jump instruction j , the jal instruction is encoded as a j -type instruction with opcode=3 as shown below.

Spring 2016, arz



ExampleLet’s compile the following procedure:

int leaf _example(int g, int h, int i, int j )

{

int f ;

f = (g+h) – (i+j );

return f ;

}

Now let’s try to compile this program using what we’ve learned sofar. To compile, the compiler must make a decision about how topass the arguments g, h, i and j to the procedure and where theprocedure must return its results to the caller.

Spring 2016, arz



Argument Passing & Return Value

The MI PS I SA does not specif y how arguments should be passed and values returned. This is done using a compiler/ assembler convention f or MI PS:

Registers 4-7 are used f or argument passing. By convention MI PS refers to these registers as $a0-$a3,

Registers 2-3 are used to return values f rom procedures. By convention MI PS refers to these registers as $v0-$v1.

Let’s assume in our example that the arguments g-i will be passed to the procedure in $a0-$a3 and the result is returned in $v0. The compiler chooses reg16 for the local variable f . I n addition the compiler uses reg8 and reg9 f or temporary storage of (g+h) & (i+j).

Spring 2016, arz



ExampleLet’s compile the f ollowing procedure:

int leaf _example(int g, int h, int i, int j )

{

int f ;

f = (g+h) – (i+j );

return f ;

}

Now let’s try to compile this program using what we’ve learned so f ar. To compile, the compiler must make a decision about how to pass the arguments g, h, i and j to the procedure and where the procedure must return its results to the caller.

$a0, $a1, $a2, $a3 = R4-R7

R16

$v0=R2

R8R9

Spring 2016, arz



What Could Go Wrong?

What if the caller also happens to be using the same registers reg8, reg9 and reg16? I n this case the values used by the caller will be overwritten by the callee and it will not be able to execute the program correctly af ter the callee returns. This problem cannot be addressed by simply using a diff erent set of registers in the caller and the callee. For one thing we don’t know how deep the nested procedure calls are. I f n procedure calls are nested, we will need

O n registers whereas we only have 32. For another this scheme

clearly will not work if the procedures are compiled separately and later linked (as in a library). The solution is to spill registers into main memory. We associate a f rame (or activation record) with a procedure call of the f unction 1 2 nf x ,x ,...x . The f rame will contain

registers that the callee plans to use during execution.

Spring 2016, arz



Call Stack The natural data structure for spilling

registers into memory is a call stack (a last-in first-out structure)

Register values are pushed and saved on the stack when the procedure is called and popped from the stack into the original register at return

Historically call stacks “grow” from High address to low address

A stack pointer is used to address the first unused memory location

MIPS software uses register 29 for stack pointer and refers to it as $sp

other machines (e.g., 80x86) may use a special-purpose stack pointer

main

Proc1

Proc2

Proc3

Proc4

Low Address

High Address

$sp

Spring 2016, arz



Carnegie Mellon

Pushing a Register on the Stack

Suppose the called procedure wants to use reg16 It must push register reg16 to save it

subi $sp, $sp, 4 Makes room for a 4-byte word on the

stack sw reg16, 0($sp)

Stores reg16 into stack memory Now the called procedure can use

reg16

-4

Stack GrowsDown

IncreasingAddresses

Stack “Bottom”

Stack Pointer: $sp

Stack “Top”

Spring 2016, arz



Stack Pointer: $sp

Stack GrowsDown

IncreasingAddresses

Stack “Top”

Stack “Bottom”

Carnegie Mellon

Popping a Register From the Stack

+4

Before the procedure returns, it must restore reg16 to original value Pops stack into register reg16

lw reg16, 0($sp) Loads reg16 from stack memory

addi $sp, $sp, 4 Pops the stack

Now the callee can use reg16 as before

Spring 2016, arz



Example - Continued

leaf_example: #this is the address in memory where the procedure is stored

#The compiler decides to use reg8, reg9 and reg16. Because the

#caller may also be using these same registers, they must be saved on the stack

subi $sp, $sp, 12 #Make room on the stack for three registers

sw reg8, 8($sp) #push reg8



add reg8, $a0, $a1 #compute g+h

add reg9, $a2,$a3 #compute i+j

sub reg16, reg8, reg9 #compute (g+h)- ( i+j)

add $v0, reg16, $zero #return result in $v0

#We are done. But before we return we must restore the registers that we decided to use

lw reg8, 8($sp) #restore reg8



addi $sp, $sp, 12 #pop the stack

j $ra #now return

Save current values on stack.

Now you can use/overwrite the registers for the procedure’s computations.

Restore old values to the registers.

Compiler assignmentsg $a0 R4h $a1 R5i $a2 R6j $a3 R7

Spring 2016, arz



$sn and $tm In the example the procedure saved and restored every register it

intended to use without knowing whether they were used by the caller. When too many registers are spilled, performance suffers.

The alternative is to define and adhere to a protocol where all procedures assume that certain registers need not be saved and restored across a procedure call. MIPS assembler conventions are:

10 registers (8-15 and 24-25) are designated as temporary registers that need not be preserved by the callee. They are referred to as $t0-$t9

if the caller uses $t0-$t9 it must save them before the call and restore them on return (caller-saved).

8 registers 16-23 are designated as saved registers that must be preserved by the callee. They are referred to as $s0-$s7

if the callee uses $s0-$s7 it must save them when the procedure is entered and restore them on return (callee saved). It doesn’t bother with $t0-$t9.

the caller will not save $s0-$s7, and the callee will not save $t0-$t9

Spring 2016, arz



Recompilation Using Register Spilling Rules

I n the example we do not need to save reg8 and reg9 but we do have to save reg16. leaf_example: #this is the address in memory where the procedure is stored

#The procedure plans to use reg8, reg9 and reg16. Following MI PS assembler conventions, the

#callee must only save reg16. This reduces register spilling improving code size and performance.

subi $sp, $sp, 4 #Make room on the stack for ONE register

sw $s0, 0($sp) #push $s0

add $t0, $a0, $a1 #compute g+h; we don’t need to save $t0 R8

add $t1, $a2,$a3 #compute i+j; we don’t need to save $t1 R9

sub $s0, $t0, $t1 #compute (g+h)- ( i+j)

add $v0, $s0, $zero #return result in $v0

#We are done. But before we return we must restore the registers that we decided to use

lw $s0, 0($sp) #restore $s0

addi $sp, $sp, 4 #pop the staclk

j $ra #now return

f $s0 R16

Compiler assignmentsg $a0 R4h $a1 R5i $a2 R6j $a3 R7

Spring 2016, arz



But There is More to Spill!

All is well if the called procedure is a leaf procedure. But when there is sequence of nested calls or a recursion we need to spill more registers. Consider compiling the f ollowing equation which computes n! recursively:

/ / f act

int f a

( 4) =

ct( int n)

{

i

4* f act( 3)

f (n< 1)

return 1;

else

ret

= 4*3* f act( 2)=4*3*2*

urn n* f act(

f act(

n- 1);

}

f act(3

1)

);

Spring 2016, arz



Compiled CodeSo let’s assume that the compiler decides to use $a0 to pass argument and $v0 for the returned value.

f act: / / no need to push saved registers on the stack. None is used.

slt $t0, $a0, 1 / / is n<1?

beq $t0, $zero, L1 / / if n>=1 go to L1

addi $v0, $zero, 1 / / return 1

j r $ra

L1: subi $a0, $a0, 1 / / n=n-1

jal f act / / compute (n-1)!; set $ra to return address

mul $v0, $v0, $a0

/ / compute n!; pretend there is a multiply instruction

j r $ra / / return to where? I nitial point of entry is lost and

/ / with what value of a0?

Now consider calling this f rom main() f or n=3:

addi $a0, $s0, 3

jal f act

Spring 2016, arz



The Problem & Solution

This is obviously not going to work because we are changing the registers $a0 and $ra across recursive calls. I n addition to overwriting argument registers which may later be needed, we overwrite $ra thus losing track of the address to which the calling procedure must eventually return. The solution again is to spill these registers by saving these registers on the call stack:

The caller saves all argument registers that it needs by pushing them on the stack before a procedure call (caller saved). I t restores them af ter the called procedure returns.

The callee saves the return address ($ra) when the procedure is entered by pushing it on the stack (callee saved). I t restores it before returning to the calling procedure.

Spring 2016, arz



Compiled Recursive Procedure

fact: #this is the address in memory where the procedure is stored

subi $sp, $sp, 8 #Make room on the stack for two registers

sw $ra, 4($sp) #push return address $ra

sw $a0, 0($sp) #push argument $a0

slti $t0, $a0, 1 #is n<1?

beq $t0, $zero, L1 #if n>=1 then go to L1

addi $v0, $zero, 1 #n<0 so return 1

addi $sp, $sp, 8 #pop the stack; don’t have to restore $ra and $a0

j $ra #now return

L1: subi $a0, $a0, 1 # n>=1. Decrement n and make recursive call with (n- 1)

jal fact

lw $a0, 0($sp) #restore original n

mul $v0, $v0, $a0 #compute n(n- 1)…..

lw $ra, 4($sp) #before we return we must restore the return address register

addi $sp, $sp, 8 #pop the stack

j $ra #now return

Spring 2016, arz



Stack Frame Pattern

Spring 2016, arz



What Else?I n our examples so f ar we assumed that the stack pointer $spdoes not change during execution of the procedure. The stackpointer is adjusted to save registers on procedure entry andreadjusted to the original value on procedure’s return. But what ifthe procedure has local variables? I n particular what if the localvariables can be declared anywhere in the body of the procedure?All of these are also created and maintained on the call stack.However as new storage is allocated on the stack the stackpointer keeps getting readjusted. I f all local variables were to bereferred to in terms of the stack pointer, the ref erence wouldhave to be readjusted each time new storage is allocated on thestack. This makes code generation cumbersome and moreimportantly the compiled code is diffi cult to understand (theoff sets keep changing).

Spring 2016, arz


The Frame PointerThe solution is to use a f rame pointer ($fp). The f rame pointerpoints to the location of the fi rst variable saved on the stack (sothat this variable has a zero off set with respect to $fp) onprocedure entry. The f rame pointer remains fixed during theprocedure’s execution; and all local variables are referred to interms of $fp. Here’s the protocol:

On entry, the procedure saves the current value of $fp onthe stack (callee saved),

On entry, the procedure sets the value of $fp to the value ofthe stack pointer $sp,

Before the procedure returns, the stack pointer is adjustedback to $sp = $fp. The procedure also restores the originalvalue of the f rame pointer $fp f rom the value saved on thestack.

Spring 2016, arz



Frame & Stack Pointers

Before the call During the call After the call

MI PS assembler uses register 30 for the f rame pointer $fp.

Spring 2016, arz



Carnegie Mellon

Frame Pointer: $fp

Stack Frames

Contents Local variables Return information Temporary space

Management Space allocated when enter

procedure “Set-up” code

Deallocated when return “Finish” code

Stack Pointer: $sp

Stack “Top”

Previous Frame

Frame for

proc

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo$fp

$sp

Stack

yooyoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

Spring 2016, arz



yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

Spring 2016, arz



yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

amI

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

amI

amI

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

amI

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

amI

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

amI(…){ • • amI(); • •}

amI(…){ • • amI(); • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo

$fp

$sp

Stack

yoo

who

yoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

who(…){ • • • amI(); • • • amI(); • • •}

who(…){ • • • amI(); • • • amI(); • • •}

Spring 2016, arz



Carnegie Mellon

Example

yoo

who

amI

amI

amI

amI

yoo$fp

$sp

Stack

yooyoo(…){ • • who(); • •}

yoo(…){ • • who(); • •}

Spring 2016, arz


Typical Microcontroller Board

Stellaris R LM3S8962 evaluation board

Spring 2016, arz


Interface Types

Parallel: multiple data lines for data Speed Short distance Examples: PCI (Peripheral Component Interconnect), ATA

(Advanced technology Attachment) Serial: single data line for data

Longer range than parallel Examples: USB, RS232, I2C, SPI, PCI-Express

Synchronous: there is a clock signal between transmitter and receiver

Examples: USB, I2C, SPI Asynchronous: no clock between transmitter and

receiver Uses START/STOP bits Examples: RS232, UART

Spring 2016, arz


General-Purpose I/O (GPIO)

Open collector circuits are used for GPIO pins

The same pin can be used for input and output

Multiple controllers can be connected to the same bus

When processor write 1 to register, the transistor is turned on and GPIO pin is pulled low

When processor write 0 to register, the transistor is turned off and GPIO pin is pulled high Wired NOR

Spring 2016, arz

CS555A – Real-Time Embedded SystemsStevens Institute of Technology

39

RS-232 Standard

RS-232 is a common interface and supports asynchronous serial connections

RS-232 is being replaced by USB

Voltage levels for an ASCII "K" character (0x4B) with 1 start bit, 8 data bits and 1 stop bit. Read this left to right corresponding to how bits are transmitted on the line:0100 1011= 0x4B.

DB9

Spring 2016, arz


Universal Asynchronous Receiver Transmitter (UART)

Converts 8-bit parallel data to serial data & vice versa The UART provides hardware support for

Parallel-to-Serial and Serial-to-Parallel conversion Start and Stop Bit framing Parity Generation Baud-Rate Generation (2400-115.2kbps)

UART supports Interrupts Transmit Complete Transmit Data Register Empty Receive Complete

Serial interface specification (RS232C) Start bit 6,7,8,9 data bits Parity bit optional Stop bit

Spring 2016, arz


UART Register Interface

CPU uses UART registers to interact with UART UDR (UART Data Register)

CPU writes byte to transmit CPU reads byte received

USR (UART Status Register) Rx/Tx complete signal bits

UCR (UART Control Register) Interrupt enable bits Rx/Tx enable bits Data format control bits (e.g. optional parity bit)

UBRR (UART Baud Rate Register) Baud rate generator division ratio

Spring 2016, arz


UART Transmission

Send a byte by writing to UDR register UART sets TXC bit in USR when the final

bit has finished transmitting UART triggers Tx Complete interrupt if

enabled in the UCR CPU must wait for current byte to finish

transmitting before sending the next one

Spring 2016, arz


UART Receive

How does the CPU know a byte has arrived? Two methods available: Polling: poll the RXC bit in USR or Interrupt: enable the Rx Complete interrupt and

write an ISR routine to handle it Read received bytes from the UDR register

Spring 2016, arz


UART Baud Rate

Set by UBRR (Baud Rate Register) UBRR (0-255) BAUD=fCK/[16*(UBRR+1) fCK is the crystal clock frequency

Spring 2016, arz


Interfacing I/O Devices

How does the CPU interface I/O devices? How is data transferred to/from memory? Role of operating system (OS)

provides system calls for accessing devices (e.g., read, write, seek)

protects one user’s data from another handles interrupts from devices

Spring 2016, arz


I/O Instructions

Addressing: the CPU must be able to address individual device’s registers

Instructions: the CPU must use instructions to send commands to I/O devices

Two techniques for I/O instructions Isolated I/O: special instructions for I/O (e.g., IN, OUT)

Different from instructions to access data memory memory-mapped I/O: same instruction set for I/O

access as for data memory access (e.g., LOAD, STORE)

Spring 2016, arz


Isolated I/O

Separate instructions for memory and I/O references IN R1, device_Address

Separate memory & I/O address space either physically separate bus (shown in diagram above) or same physical bus with a signal to indicate memory or I/O

Used in Intel 80x86

CPU

CPU

Memory

Memory

Interface

I/OPeripheral

I/OPeripheral

Interface

I/OPeripheral

I/OPeripheral

Pro

cessor-

mem

ory

bu

s

Independent I/O bus

Spring 2016, arz


Memory-Mapped I/O

Common memory & I/O bus Same instruction set for memory access & I/O

e.g., LOAD R1, 0(R5): R5 maps to an external I/O register Same address space for memory & I/O More prevalent than isolated I/O: used in RISC processors

CPU

CPU

Memory

Memory

Interface

I/OPeripheral

I/OPeripheral

Interface

I/OPeripheral

I/OPeripheral

Common Memory & I/O bus

ROM

RAM

I/O

Spring 2016, arz


Polling Main loop uses each I/O device periodically If output is to be produced, produce it If input is ready, read it

Example:USR (UART Status Register)

Rx/Tx complete signal bits

Spring 2016, arz


Send on a Polled UART I/0

Loop until TX buffer is empty (6th bit of Status register is set to 1)

Write Data register with your data.

Spring 2016, arz


Send a Byte Sequence

The lower the I/O speed the more CPU cycles are wasted. As CPU clock rate increases, there is more in polling penalty.

(8/57600)*(18000000)=2500 cycles

Spring 2016, arz


Receive With Polling

Loop until RX buffer is full (8th bit of Status register is set to 1)

Why?

Spring 2016, arz


Interrupt-Driven I/O

External hardware alerts the processor that input is ready Processor suspends what it is doing Processor invokes an interrupt service routine (ISR) ISR interacts with the application concurrently

Spring 2016, arz


Control Flow in Absence of Interrupts

<startup>inst1

inst2

inst3

…instn

<shutdown>

Processors do only one thing: From startup to shutdown, a CPU simply reads and executes

(interprets) a sequence of instructions, one at a time This sequence is the CPU’s control flow

Physical control flow

Time

Spring 2016, arz


Altering the Control Flow

Up to now: two mechanisms for changing control flow: Jumps and branches Call and returnBoth react to changes in state within the program

Insufficient for a useful system: A useful system must also react to changes in system state (CPU + peripherals)

data arrives from a disk or a network adapter user hits Ctrl-C at the keyboard System timer expires

System needs mechanisms for “exceptional control flow”

Spring 2016, arz


Exceptional Control Flow

Exceptional events exist at all levels of a computer system

Low level mechanisms Change in control flow in response to a system event

(i.e., divide by zero) Hardware interrupts

Higher level mechanisms Process context switch OS system calls

Exception categories Asynchronous Synchronous

Spring 2016, arz


Asynchronous Exceptions (Interrupts)

Caused by events external to the processor Indicated by asserting the processor’s interrupt pin

which is examined by the processor after executing each instruction

Processor completes execution of “current” instruction

Interrupt handler returns to “next” instruction in the original code

Examples: I/O interrupts

Arrival of a packet from network Arrival of data from a disk

Hard reset interrupt Hitting the reset button

Spring 2016, arz


External Exception Interface

ProcessorProcessorPriorityencoderPriorityencoder

Level 1

Level 2

Level 3

Level 4

Level 5

Level 6

Level 7

I0

I1

I2

000: no interrupt010: level 2 int.

Spring 2016, arz


Synchronous Exceptions Caused by events that occur as a result of executing an

instruction: Traps

Intentional and planted by design Examples: system calls, breakpoint traps, special instructions After handling, control must return to “next” instruction

Faults Unintentional but possibly recoverable Examples: page faults (recoverable), protection faults

(unrecoverable), floating point exceptions Either re-executes faulting (“current”) instruction or aborts

Aborts Unintentional and unrecoverable Examples: memory parity error Aborts current program

Spring 2016, arz


Exception Handling

Triggers A level change on an interrupt request pin Software writing to an interrupt control register

(“software interrupt”), causing a level change in an interrupt pin

Executing a special “SysCall” instruction Responses

Disable interrupts Push the program counter into the special Exception

Program Counter (EPC) Execute Interrupt Service Routine (ISR) instructions

beginning at a designated address in memory Design of interrupt service routine

Save and restore any registers it uses Re-enable interrupts before returning from ISR

Spring 2016, arz


Saving the Processor State

Address Instruction

0x1230 add R0, R1, R10x1234 div R3, R2, R00x1238 add R1, R1, R20x123C addi R2, R2, 1

PC: 0x1234

Exception PC (EPC): 0x1234

CauseValue

• All actions performed byprocessor before enteringexception service routine• Interrupts disabled

Spring 2016, arz


Invoking Exception Service Routine

ExceptionHandler0x80000080

Instruction memory

EPC: 0x1234

Address Instruction

0x1230 add R0, R1, R10x1234 div R3, R2, R00x1238 add R1, R1, R20x123C addi R2, R2, 1

PC: 0x80000080

CauseValue

Spring 2016, arz


Exception Service Routine

ExceptionHandler(){if (cause == Arithmetic Overflow) ArithmeticOverflowHandler();else if (cause == DivideByZero) DivideByZeroHandler();else if (cause == Illegal Instruction) IllegalInstructionHandler();else if (cause == external interrupt) InterruptHandler();……………………………….}

Cause Register

00cause

Overflow12

Breakpoint9

Store addr. error5

Load addr. error4

Ext. interrupt0

Example Cause Values

Spring 2016, arz


Trap Example: Opening File

User calls: int open(filename, options) Function open executes system call instruction via __libc_open

OS must find file, get it ready for reading or writing Returns integer file descriptor as a handle to the user

User Process OS

exception

open filereturns

intpop

Spring 2016, arz


Fault Example: Page Fault

User attempts write to a memory location That portion (page) of user’s memory

is currently on disk

Page handler must load page into physical memory Returns to faulting instruction Successful on second try

int a[1000];main (){ a[500] = 13;}

User Process OS

exception: page faultCreate page and load into memoryreturns

store

Spring 2016, arz


Abort Example: Invalid Memory Reference

Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with “segmentation fault”

int a[1000];main (){ a[5000] = 13;}

User Process OS

exception: page fault

detect invalid address

store

signal process

Suppose address 5000 has not been mapped

Spring 2016, arz


External Timers

Programmable Interval timer (PIT) Counts down from some value to

zero and then triggers an interrupt The initial timer value is set by

writing to a memory-mapped register

It can be configured to trigger repeatedly by HW without software ISR restarting it

Spring 2016, arz


Spring 2016, arz


Spring 2016, arz


Volatile Keyword Use

• An optimizing compiler decidesthat no one in the body of the Program is changing foo.• So it transforms the programto an infinite loop.• But foo may be a memory-mappedI/O or changed by an interrupt routine. It may change external to the program.

The volatile keyword tells the compliernot to optimize this code. Compiler leavesthe code unchanged.

Spring 2016, arz


Spring 2016, arz


Spring 2016, arz


Spring 2016, arz


Spring 2016, arz


Decrementing unsigned int

Spring 2016, arz


#include "stdio.h"

int main(void){ unsigned int x=0; x--; printf("Hello world: %u!\n", x);}

Hello world: 4294967295!

Spring 2016, arz


User Mode vs. System (aka Privileged or Kernel) Mode

Operating system kernel executes in the privileged mode

has unrestricted access to all system resources protects user programs from each other (e.g.,

memory protection) protects system against malicious use (all user access

to system resources is via system calls) User programs run in user mode with controlled

access to system resources via system calls Exception handling is done in system mode

because unrestricted access is typically required

Spring 2016, arz


The Path Of I/O Transfer

In both polled I/O & interrupt-driven I/O, the path for data transfer is through the processor registers

For high-performance systems and high-bandwidth I/O peripherals both techniques are inefficient

Alternative: Direct-Memory Access (DMA) removes the processor from the data transfer path

a limited form of multiprocessing (DMA is a specialized processor)

Common Memory & I/O bus

RegistesRegistes

Processor

ROM

RAM

I/OLOAD

STORE

Mem

ory

-map

ped

I/O

Spring 2016, arz


I/O Using DMA

CPU sends device name, address, length and transfer direction to DMA controller (via memory-mapped I/O)

CPU issues start command to DMA controller DMA controller provides handshake signals to I/O device &

memory including addresses DMA controller interrupts processor when transfer is complete

CPU

CPU

Memory

Memory

Interface

I/OPeripheral

I/OPeripheral

Interface

I/OPeripheral

I/OPeripheral

ROM

RAM

I/O DMAController

DMAController

DMA

Mem

ory

-map

ped

I/O Data

transferControl

Documents

CPE555A: Real-Time Embedded Systems