Chapter 8 I/O Programming

Chapter 9Trap Service Routines

• Programmed I/O

• Interrupts

• Interrupt Driven I/O

• Trap Service Routines

Input/Output Addressing

• Memory Mapped I/O

A section of the memory address space is reserved for I/O Registers rather than general memory locations. Think of it as “pseudo” memory. The same instructions are used for general programming and I/O programming.

• Non-Memory Mapped I/O

There is a separate address space for I/O programming, and an entirely separate set of I/O Instructions.

Synchronous vs Asynchronous I/O

• Synchronous

Latest value of data could be expected to be available when the program wanted it.

It might be periodically updated at a known frequency.

This is not typical nor usually realistic for I/O.

• Asynchronous

Computer is generally much faster than I/O so program must wait until requested data is available or data provided has been taken.

“Handshaking” is used to ensure that data is available or I/O device is ready.

LC-3 Memory Map

(64K of 16 bit words)

256 words

256 words

23.5 K words

39.5 K words

512 words

LC-3 has Memory Mapped I/O

LC-3 Memory Layout:x0000 – x00FF Trap vectors (Supports Software Interrupts) x0020 [x0400] GETC (Read Char from Keyboard)

x0021 [x0430] OUT (Write Character to Console)

x0022 [x0450] PUTS (Write string to Console)

x0023 [x04A0] IN (Prompt, input character from Keyboard, echo character to Console)

x0024 [x04E0] PUTSP (Write “packed” string to Console)

x0025 [xFD70] HALT (Turn off run latch in MCR)

x0100 – x01FF Interrupt Vectors (Supports Hardware Interrupts)

x0200 – x2FFF System Programs & Data (“Operating System”)

x3000 – xFDFF User Programs Area

xFE00 – xFFFF I/O Programming “Registers” (Mapped I/O Regs)

xFE00 KBSR [15 {Ready}, 14 {Intr enable}] (Keyboard Status Register)

xFE02 KBDR [7:0{ascii data}] (Keyboard Data Register)

xFE04 DSR [15{Done}, 14{Intr enable}] (Display Status Register)

xFE06 DDR [7:0{ascii data}] (Display Data Register

xFFFE MCR [15{Run latch}] (Machine Control Register)

The LC-3 Computer & I/O

Memory

PSW (Program Status Word): Bits: 15 10 9 8 2 1 0 | S| |Priority| | N| Z| P|

PSW

I/O

LC-3 Memory Mapped I/O

Polling vs Interrrupt Driven I/O

• Polling I/O

Program initiates I/O and then checks regularly to see if I/O is completed. (typically a loop in the program)

• Interrupt Driven I/O

Program initiates I/O and goes to sleep until the CPU wakes up the program to say that I/O is completed. (instead of going to sleep, the CPU may do something else before the wake up call)

Handshaking Communications

• A Paradigm Used in Asynchronous Communications

• Typical sequence:– Initiate I/O Request (send signal to I/O device)– Loop checking for I/O complete (check for returned

signal)

– Continue Program (perhaps Initiate another I/O Request)– Etc.

Keyboard Input Interface

Keyboard Input Registers

KBDR (Keyboard Data Register): Assigned to address xFE02

Data is in KBDR[7:0] Read only Register

KBSR (Keyboard Status Register): Assigned to address xFE00

Status “ready” is KBSR[15]

Set to “1” when new data is ready (Set by Keyboard)

Cleared when data is read (Cleared when user reads data)

Simple Polling Program for Input from Keyboard

; Waiting for a character to be entered

START LDI R1, KBSR ; Test for character input

BRzp START ; If not here, try again

LDI R0, KBDR ; Read character

BRnzp NEXT_TASK ; Go to the next task

~

~

KBSR .FILL xFE00 ; Address of KBSR (Keyboard Status Register)

KBDR .FILL xFE02 ; Address of KBDR (Keyboard Data Register)

xFE02

xFE00

Console Output Interface

Console (Monitor) Output Registers

DDR (Display Data Register): Assigned to Address xFE06

Data is in DDR[7:0]

DSR (Data Status Register): Assigned to Address xFE04

Status “done” bit is DSR[15]

Set to “1” when data is picked up

Cleared when new data is written

Simple Polling Program to Output to Console

; Wait for write to Console done

START LDI R1, DSR ; Test for output written

BRzp START ; If not done, try again

STI R0, DDR ; Write character

BRnzp NEXT_TASK ; Go to next task

~

~

DSR .FILL xFE04 ; Address of DSR (Display Status Register)

DDR .FILL xFE06 ; Address of DDR (Display Data Register)

FE06

FE04

Program to Echo from Keyboard to Monitor

; Echo keyboard input to Console output

START LDI R1, KBSR ; Test for input ready

BRzp START

LDI R0, KBDR

ECHO LDI R1, DSR ; Test for output done

BRzp ECHO

STI R0, DDR

BRnzp NEXT_TASK

KBSR .FILL xFE00 ; Address of KBSR

KBDR .FILL xFE02 ; Address of KBDR

DSR .FILL xFE04 ; Address of DSR

DDR .FILL xFE06 ; Address of DDR

A “Complete” I/O Routine to Echo a Line of Input .orig x3000

; Program to read and echo line from the Console

ST R1, SaveR1 ; Save registers needed ST R2, SaveR2 ; by this routine ST R3, SaveR3

LD R2, Newline ; Store newline Character in R2

L1 LDI R3, DSR ; Loop until Monitor is done BRzp L1 STI R2, DDR ; Move cursor to new clean line

LEA R1, Prompt ; Load starting address of prompt stringLoop LDR R0, R1, #0 ; Get prompt character BRz Input ; Branch to Loop on null (0)

L2 LDI R3, DSR ; Loop until Monitor is done BRzp L2 STI R0, DDR ; Write prompt character ADD R1, R1, #1 ; Increment Prompt pointer BRnzp Loop ; Go to get next prompt character

A “Complete” I/O Routine to Echo a Line of Input (2)

Input LDI R3, KBSR ; Poll until a character is typed BRzp Input LDI R0, KBDR ; Get input character

L3 LDI R3, DSR ; Loop until Monitor is done BRzp L3 STI R0, DDR ; Echo input character

ADD R0, R0, #-10 ; Test for newline (done) BRnp Input ; Loop if not done

L4 LDI R3, DSR ; Loop until Monitor is done BRzp L4 STI R2, DDR ; Move cursor to new clean line

LD R1, SaveR1 ; Restore registers LD R2, SaveR2 ; to original values LD R3, SaveR3

BRnzp NEXT_TASK ; Do the program's next task

A “Complete” I/O Routine to Echo a Line of Input (3)

SaveR1 .BLKW 1 ; Memory for registers savedSaveR2 .BLKW 1 SaveR3 .BLKW 1

DSR .FILL xFE04DDR .FILL xFE06KBSR .FILL xFE00KBDR .FILL xFE02

Newline .FILL x000A ; ASCII code for newline

Prompt .STRINGZ "Input character line> "

NEXT_TASK BRnzp NEXT_TASK ; Simulates next task.END

I/O Interrupts

Requirements for a device to interrupt the processor

• The device must have the right to request service

• The I/O device must want service

• The device request must be at a higher priority than what is being done by the processor or is being requested by other devices

• The processor must be completed with the present instruction execution

Device(s) Generating Interrupt Service Request to the CPU

Keyboard:

Monitor:

Done/Ready bit is “anded” with the Interrupt Enable bit to produce an interrupt request signal

Generating the LC-3 Interrupt Request to the CPU

Interrupt Vectors are: x0100 to x01FF (x0180 is for the Keyboard)

Servicing an Interrupt

The following process is followed to service an interrupt:• The CPU enters the Supervisor State (PSW Bit 15 set to 0)

• The “context” of the present program is saved (PC, PSW, SP) (Why?)

• The device provides the address of location in the interrupt service routine table where the pointer to the service routine should reside. (x0180 is the Interrupt Vector for the Keyboard Interrupt Service Routine)

• The Supervisor loads the address of the service routine into the PC

• The service routine is executed ending with an RTI (Any registers used will be saved and restored)

• The context of the original program is reloaded, and • The original program resumes

Trap Routines

• Programming Trap Service Routines

– Writing the code for them

Trap Instruction:

TRAP x

1111 0000 trap vector F0xx

[PC ] R7

Jump to routine at trap vector address

Return:

RET

1100 000 111 000000 C1C0

[R7] PC (JMP R7)

Some Conventions (For the LC-3):

Data passed (read & write) in R0,

Error messages returned in R5

Trap Routines

Trap Process

1) Execute TRAP “vector” - Service Routine Addresses

Trap Vectors are at memory locations [0000:00FF]

Trap Vectors contain addresses of “Pre written (?)” Service Routines

2) [PC] is stored in R7

3) Address of Trap Service Routine loaded into PC

4) Service Routine Program executed

5) Trap service routine program ends with an RET ([R7] loaded into PC)

TRAP x21 OUT Trap Vector Routine(Output Character)

; out.asm;.ORIG x0430 ; System call starting address ST R1, SaveR1 ; R1 will be used to poll the DSR ; hardware;; Write the character;TryWrite LDI R1, DSR ; Get status BRzp TryWrite ; Bit 15 on says display is readyWriteIt STI R0, DDR ; Write character;; Return from trap;Return LD R1, SaveR1 ; Restore registers RET ; Return from trap (JMP R7, actually);DSR .FILL xFE04 ; Address of display status registerDDR .FILL xFE06 ; Address of display data registerSaveR1 .BLKW 1 ; Save area for R1 .END

; in.asm Service Routine for Keyboard Input;.ORIG x04A0START ST R1,SaveR1 ; Save the register values ST R2,SaveR2 ; that are used so that they ST R3,SaveR3 ; can be restored before RET; LD R2,NewlineL1 LDI R3,DSR ; Check DDR -- is it free? BRzp L1 STI R2,DDR ; Move cursor to new clean line; LEA R1,Prompt ; Prompt is starting address ; of prompt stringLoop LDR R0,R1,#0 ; Get next prompt character BRz Input ; Check for end of prompt stringL2 LDI R3,DSR BRzp L2 STI R0,DDR ; Write next character of ; prompt string ADD R1,R1,#1 ; Increment Prompt pointer BRnzp Loop;Input LDI R3,KBSR ; Has a character been typed? BRzp Input LDI R0,KBDR ; Load it into R0L3 LDI R3,DSR BRzp L3 STI R0,DDR ; Echo input character ; to the monitor;L4 LDI R3,DSR BRzp L4 STI R2,DDR ; Move cursor to new clean line LD R1,SaveR1 ; Service routine done, restore LD R2,SaveR2 ; original values in registers. LD R3,SaveR3 RET ; Return from trap (i.e., JMP R7)

TRAP x23 IN Trap Service Routine (Input Character)

;SaveR1 .BLKW 1SaveR2 .BLKW 1SaveR3 .BLKW 1DSR .FILL xFE04DDR .FILL xFE06KBSR .FILL xFE00KBDR .FILL xFE02Newline .FILL x000A ; newline (ASCII) Prompt .STRINGZ "Input a character>".END

TRAP x25 HALT Service Routine ; halt.asm Halts the program;.ORIG xFD70 ; Where this routine resides ST R7, SaveR7 ST R1, SaveR1 ; R1: a temp for MC register ST R0, SaveR0 ; R0 is used as working space

; print message that machine is halting

LD R0, ASCIINewLine TRAP x21 LEA R0, Message TRAP x22 LD R0, ASCIINewLine TRAP x21;; clear bit 15 at xFFFE to stop the machine; LDI R1, MCR ; Load MC register into R1 LD R0, MASK ; R0 = x7FFF AND R0, R1, R0 ; Mask to clear the top bit STI R0, MCR ; Store R0 into MC register;; return from HALT routine. ; (how can this routine return if the machine is halted above?); LD R1, SaveR1 ; Restore registers LD R0, SaveR0 LD R7, SaveR7 RET ; JMP R7, actually;; Some constants;ASCIINewLine .FILL x000ASaveR0 .BLKW 1SaveR1 .BLKW 1SaveR7 .BLKW 1Message .STRINGZ "Halting the machine."MCR .FILL xFFFE ; Address of MCRMASK .FILL x7FFF ; Mask to clear the top bit .END

TRAP 22 PUTS Trap Service Routine(Output a Character String)

; puts.asm Output a Character String

; This service routine writes a NULL-terminated string to the console.; It services the PUTS service call (TRAP x22). ; Inputs: R0 is a pointer to the string to print.; Context Information: R0, R1, and R3 are saved, and R7 is lost; in the jump to this routine;.ORIG x0450 ; Where this Service Routine resides ST R7, SaveR7 ; Save R7 for later return ST R0, SaveR0 ; Save other registers that are used by this routine ST R1, SaveR1 ST R3, SaveR3 ;; Loop through each character in the array;;Loop LDR R1, R0, #0 ; Retrieve the character(s) BRz Return ; If it is 0, doneL2 LDI R3,DSR BRzp L2 STI R1, DDR ; Write the character ADD R0, R0, #1 ; Increment pointer BRnzp Loop ; Do it all over again;; Return from the request for service callReturn LD R3, SaveR3 ; Restore Registers LD R1, SaveR1 LD R0, SaveR0 LD R7, SaveR7 RET;; Register locationsDSR .FILL xFE04DDR .FILL xFE06

SaveR0 .FILL x0000 SaveR1 .FILL x0000SaveR3 .FILL x0000SaveR7 .FILL x0000

.END