1. Introduction to Computer Engineering
CS/ECE 252, Fall 2016
Kai Zhao
Computer Sciences Department
University of Wisconsin – Madison

2. Chapter 8 & 9.1 I/O and Traps Quote of the Day:
“Success consists of going from failure to failure without loss of enthusiasm”
-- Sir Winston Churchill

3. I/O: Connecting to Outside World
So far, we’ve learned how to:
compute with values in registers
alter the sequence of instructions
load data from memory to registers
store data from registers to memory
But where does data in memory come from?
And how does data get out of the system so that humans can use it?
Operate instructions
Control Flow Instructions
Data Movement Instructions

4. I/O: Connecting to the Outside World
Types of I/O devices characterized by:
behavior: input, output, storage
input: keyboard, motion detector, network interface
output: monitor, printer, network interface
storage: disk, CD-ROM
data rate: how fast can data be transferred?
keyboard: 100 bytes/sec
disk: 30 MB/s
network: 1 Mb/s - 1 Gb/s
Input: mouse, trackpad, joystick, game controllers

5. Aside Off-Topic: Memory Hierarchy

6. I/O Controller CPU Control/Status Registers Data Registers
CPU tells device what to do -- write to control register
CPU checks whether task is done -- read status register
Data Registers
CPU transfers data to/from device
Device electronics
performs actual operation
pixels to screen, bits to/from disk, characters from keyboard
Graphics Controller
Control/Status
CPU
Electronics
display
Output Data

7. Programming Interface
How are device registers identified?
Memory-mapped vs. special instructions
How is timing of transfer managed?
Asynchronous vs. synchronous
Who controls transfer?
CPU (polling) vs. device (interrupts)

8. Memory-Mapped vs. I/O Instructions
designate opcode(s) for I/O
register and operation encoded in instruction
Memory-mapped
assign a memory address to each device register
use data movement instructions (LD/ST) for control and data transfer
IO = opcode for IO, device = keyboard/display, Op = read/write
What? Memory and I/Odevice are 2 different devices. What does it mean to write to memory and have it show up on output device monitor?

9. Transfer Timing
I/O events generally happen much slower than CPU cycles.
Synchronous
data supplied at a fixed, predictable rate
CPU reads/writes every X cycles
Asynchronous
data rate less predictable
CPU must synchronize with device, so that it doesn’t miss data or write too quickly
Synchronous is like a scientist that records the temperature of water every hour.
Asynchronous is like a scientist recording the tempura whenever he or she feels like it.

10. Transfer Control Who determines when the next data transfer occurs?
Polling
CPU keeps checking status register until new data arrives OR device ready for next data
“Are we there yet? Are we there yet? Are we there yet?”
Interrupts
Device sends a special signal to CPU when new data arrives OR device ready for next data
CPU can be performing other tasks instead of polling device.
“Wake me when we get there.”
Example of polling with asking questions in lectures

11. LC-3 Memory-mapped I/O (Table A.3) xFE00 xFE02 xFE04
Asynchronous devices
synchronized through status registers
Polling and Interrupts
the details of interrupts will be discussed in Chapter 10
Location
I/O Register
Function
xFE00
Keyboard Status Reg (KBSR)
Bit [15] is one when keyboard has received a new character.
xFE02
Keyboard Data Reg (KBDR)
Bits [7:0] contain the last character typed on keyboard.
xFE04
Display Status Register (DSR)
Bit [15] is one when device ready to display another char on screen.
xFE06
Display Data Register (DDR)
Character written to bits [7:0] will be displayed on screen.
For this course, we will only use polling

12. Input from Keyboard When a character is typed: When KBDR is read:
its ASCII code is placed in bits [7:0] of KBDR (bits [15:8] are always zero)
the “ready bit” (KBSR[15]) is set to one
keyboard is disabled -- any typed characters will be ignored
When KBDR is read:
KBSR[15] is set to zero
keyboard is enabled
keyboard data 15 8 7
KBDR 15 14
ready bit
KBSR

13. Basic Input Routine POLL LDI R0, KBSRPtr BRzp POLL LDI R0, KBDRPtr ...
KBSRPtr .FILL xFE00 KBDRPtr .FILL xFE02
new
char? NO
Polling
YES
read character

14. Simple Implementation: Memory-Mapped Input
Address Control Logic
determines whether
MDR is loaded from Memory or from KBSR/KBDR.
Addresses xFE00 or xFE02
Location
I/O Register
xFE00
Keyboard Status Reg (KBSR)
xFE02
Keyboard Data Reg (KBDR)

15. Output to Monitor When Monitor is ready to display another character:
the “ready bit” (DSR[15]) is set to one
When data is written to Display Data Register:
DSR[15] is set to zero
character in DDR[7:0] is displayed
any other character data written to DDR is ignored (while DSR[15] is zero)
output data 15 8 7
DDR 15 14
ready bit
DSR

16. Basic Output Routine POLL LDI R1, DSRPtr BRzp POLL STI R0, DDRPtr ...
DSRPtr .FILL xFE04 DDRPtr .FILL xFE06
screen
ready? NO
Polling
YES
write character

17. Simple Implementation: Memory-Mapped Output
Selecting DSR as input is used to determine whether device is ready to write
Setting DDR is for display output.
1. The MAR is loaded with the address of the memory location to be written.
2. The MDR is loaded with the data to be written to memory.
3. Memory is written, resulting in the contents of MDR being stored in the specified memory location.
Sets LD.DDR
or selects
DSR as input.

18. Keyboard Echo Routine
Usually, input character is also printed to screen.
User gets feedback on character typed and knows its ok to type the next character.
new
char?
POLL1 LDI R0, KBSRPtr BRzp POLL1 LDI R0, KBDRPtr POLL2 LDI R1, DSRPtr BRzp POLL2 STI R0, DDRPtr
...
KBSRPtr .FILL xFE00 KBDRPtr .FILL xFE02 DSRPtr .FILL xFE04 DDRPtr .FILL xFE06 NO
YES
read character
GETC and OUT
screen
ready? NO
YES
write character

19. In-Class Exercise output data DDR 2014Fall Exam4 Q515 8 7
DDR
2014Fall Exam4 Q5
Complete the code snippet below to display the string. .ORIG x3000
LEA R3, STRING
NEXT LDR R0, R3, #0
______________________
______________________ ; TODO
ADD R3, R3, #1 ; increment pointer to next char
BR NEXT
END HALT
STRING .STRINGZ "Have a Happy Thanksgiving!" ; string to print
DSR .FILL xFE04
DDR .FILL xFE06
.END 15 14
ready bit
DSR
Quote of the Day:
“Do what you can, with what you have, where you are.”
-- Theodore Roosevelt, US President

20. In-Class Exercise Solutions
output data
In-Class Exercise Solutions 15 8 7
DDR
2014Fall Exam4 Q5
Complete the code snippet below to display the string. .ORIG x3000
LEA R3, STRING
NEXT LDR R0, R3, #0
BRz END
POLL LDI R1, DSR
BRzp POLL
STI R0, DDR
ADD R3, R3, #1 ; increment pointer to next char
BR NEXT
END HALT
STRING .STRINGZ "Have a Happy Thanksgiving!" ; string to print
DSR .FILL xFE04
DDR .FILL xFE06
.END 15 14
ready bit
DSR

21. Interrupt-Driven I/O External device can:
Force currently executing program to stop;
Have the processor satisfy the device’s needs; and
Resume the stopped program as if nothing happened.
Why?
Polling consumes a lot of cycles, especially for rare events – these cycles can be used for more computation.
Example: Process previous input while collecting current input. (See Example 8.1 in text.)

22. Interrupt-Driven I/O To implement an interrupt mechanism, we need:
A way for the I/O device to signal the CPU that an interesting event has occurred.
A way for the CPU to test whether the interrupt signal is set and whether its priority is higher than the current program.
Generating Signal
Software sets "interrupt enable" bit in device register.
When ready bit is set and IE bit is set, interrupt is signaled.
interrupt enable bit 15 14 13
ready bit
KBSR
interrupt signal to processor

23. Priority Every instruction executes at a stated level of urgency.
LC-3: 8 priority levels (PL0-PL7)
Example:
Payroll program runs at PL0.
Nuclear power plant control program runs at PL6.
It’s OK for PL6 device to interrupt PL0 program, but not the other way around.
Priority encoder selects highest-priority device, compares to current processor priority level, and generates interrupt signal if appropriate.
We won’t be dealing with interrupts

24. Priority Encoder 4 to 2 Simple Encoder 4 to 2 Priority Encoder I3 I21
4 to 2 Priority Encoder I3 I2 I1 I0 O1 O0 1 x

25. Testing for Interrupt Signal
CPU looks at signal between STORE and FETCH phases.
If not set, continues with next instruction.
If set, transfers control to interrupt service routine. F NO D
interrupt
signal?
Transfer to
ISR
YES EA
ISR = interrupt service routine OP EX
More details in Chapter 10. S

26. Full Implementation of LC-3 Memory-Mapped I/O
Because of interrupt enable bits, status registers (KBSR/DSR) must be written, as well as read.

27. In-Class Exercise
The 2 methods device registers are 1) memory-mapped and 2) special instructions. What is the difference between memory-mapped and special instructions?
The 2 methods of transfer timing is 1) synchronous and 2) asynchronous. When would you use one over the other?
The 2 methods of data transfer is 1) polling and 2) interrupts. Which is better if data transfer occurs frequently?
Quote of the Day:
“What you're thinking is what you're becoming.”
- Muhammad Ali

28. Chapter 9.1: System Calls
Certain operations require specialized knowledge and protection:
specific knowledge of I/O device registers and the sequence of operations needed to use them
I/O resources shared among multiple users/programs; a mistake could affect lots of other users!
Not every programmer knows (or wants to know) this level of detail
Provide service routines or system calls (part of operating system) to safely and conveniently perform low-level, privileged operations
So that programmer doesn’t have to program for every keyboard model. Different devices have different sequences of operation (status then data).
Furthermore, we want to make sure programs have the correct permissions to access I/O device. So use system call.
Programmer usually don’t want to deal with I/O, which keyboard the user is using.

29. In LC-3, this is done through the TRAP mechanism.
System Call
1. User program invokes system call.
2. Operating system code performs operation.
3. Returns control to user program.
In LC-3, this is done through the TRAP mechanism.

30. LC-3 TRAP Mechanism 1. A set of service routines.
part of operating system -- routines start at arbitrary addresses (convention is that system code is below x3000)
up to 256 routines
2. Table of starting addresses.
stored at x0000 through x00FF in memory
called System Control Block in some architectures
3. TRAP instruction.
used by program to transfer control to operating system
8-bit trap vector names one of the 256 service routines
4. A linkage back to the user program.
want execution to resume immediately after the TRAP instruction

31. TRAP Instruction Trap vector Where to go How to get back
identifies which system call to invoke
8-bit index into table of service routine addresses
in LC-3, this table is stored in memory at 0x0000 – 0x00FF
8-bit trap vector is zero-extended into 16-bit memory address
Where to go
lookup starting address from table; place in PC
How to get back
save address of next instruction (current PC) in R7

32. TRAP
NOTE: PC has already been incremented during instruction fetch stage.

33. TRAP Example PC is currently 0x4000 mem[x0023] is currently x04A0
PC is currently 0x4000
mem[x0023] is currently x04A0
What is the contents of R7 and PC after the following instruction is executed?
x0023
x4001
x0023
x23
x04A0
x04A0
NOTE: PC has already been incremented during instruction fetch stage.

34. RET (JMP R7)
How do we transfer control back to instruction following the TRAP?
We saved old PC in R7.
JMP R7 gets us back to the user program at the right spot.
LC-3 assembly language lets us use RET (return) in place of “JMP R7”.
Must make sure that service routine does not change R7, or we won’t know where to return.

35. RET Example RET RET PC is currently 0x0220 R7 is currently 0x4001
What is the contents of PC after the following instruction is executed?
RET
RET
0x4001

36. TRAP Mechanism Operation
Lookup starting address.
Transfer to service routine.
Return (JMP R7).
Remember to load the os

37. Example: Using the TRAP Instruction
.ORIG x3000
LD R2, TERM ; Load negative ASCII ‘7’ LD R3, ASCII ; Load ASCII difference AGAIN TRAP x23 ; input character ADD R1, R2, R0; Test for terminate BRz EXIT ; Exit if done ADD R0, R0, R3; Change to lowercase TRAP x21 ; Output to monitor... BRnzp AGAIN ; ... again and again... TERM .FILL xFFC9 ; -‘7’ ASCII .FILL x0020 ; lowercase bit EXIT TRAP x25 ; halt .END
Keep outputing lower case to monitor until 7 occurs

38. Example: Output Service Routine
.ORIG x0430 ; syscall address ST R7, SaveR7 ; save R7 & R1 ST R1, SaveR1 ; Write character TryWrite LDI R1, CRTSR ; get status BRzp TryWrite ; look for bit 15 on WriteIt STI R0, CRTDR ; write char ; Return from TRAP Return LD R1, SaveR1 ; restore R1 & R7 LD R7, SaveR7 RET ; back to user CRTSR .FILL xF3FC CRTDR .FILL xF3FF SaveR1 .FILL 0 SaveR7 .FILL 0 .END
stored in table, location x21

39. Assembler Names and their TRAP Routines
Code
Equivalent
Description
GETC
TRAP x20
Read one character from keyboard. Character stored in R0[7:0].
OUT
TRAP x21
Write one character (in R0[7:0]) to console.
PUTS
TRAP x22
Write null-terminated string to console. Address of string is in R0. IN
TRAP x23
Print prompt on console, read (and echo) one character from keybd. Character stored in R0[7:0].
HALT
TRAP x25
Halt execution and print message to console.
Quote of the Day:
“Money doesn't buy happiness, yet everybody would like to find out if it is true.”
- Stefan Kisielewski (1911 – 1991) Polish writer, publicist, composer and politician

40. Basic Input Routine new char? Polling read character NO YES
Overwriting your trap vector table is bad. Might no longer respond to IO.
USB device remains lit after powering down, so I went into USB detect in motherboard settings.

41. Saving and Restoring Registers
Must save the value of a register if:
Its value will be destroyed by service routine, and
We will need to use the value after that action.
Who saves?
caller of service routine?
knows what it needs later, but may not know what gets altered by called routine
called service routine?
knows what it alters, but does not know what will be needed later by calling routine

42. What’s wrong with this TRAP x23 usage? What happens to R7?
Example
LEA R3, Binary LD R6, ASCII ; char->digit template LD R7, COUNT ; initialize to 10 AGAIN TRAP x23 ; Get char ADD R0, R0, R6 ; convert to number STR R0, R3, #0 ; store number ADD R3, R3, #1 ; incr pointer ADD R7, R7, -1 ; decr counter BRp AGAIN ; more? BRnzp NEXT ASCII FILL xFFD0 COUNT FILL #10 Binary .BLKW #10
This reads 10 characters from keyboard and store to memory
We overwrite R7

43. Saving and Restoring Registers
Called routine -- “callee-save”
Before start, save any registers that will be altered (unless altered value is desired by calling program!)
Before return, restore those same registers
Calling routine -- “caller-save”
Save registers destroyed by own instructions or by called routines (if known), if values needed later
save R7 before TRAP
save R0 before TRAP x23 (input character)
Or avoid using those registers altogether
Values are saved by storing them in memory.

44. Summary Chapter 8: Input/output Chapter 9: Traps and System Calls
Behavior and data rate of I/O device
Asynchronous vs. synchronous
Polled vs. interrupt-driven
Programmed vs. memory-mapped
Control registers, data registers
Chapter 9: Traps and System Calls
Hide details of I/O device interaction
TRAP/RET instructions
Caller- vs callee-saved registers