Computer Science 210 s1c
Computer Systems 1
2014 Semester 1
Lecture Notes
Lecture 10
Machine language programs
James Goodman (revised by Robert Sheehan)
Credits: “McGraw-Hill” slides prepared by Gregory T. Byrd, North Carolina State University1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Using Branch Instructions
Compute sum of 12 integers.
Numbers start at location x3100. Program starts at location x3000.
R1  x3100
R3  0
R2  12
R2=0?
NO
R4  M[R1]
R3  R3+R4
R1  R1+1
R2  R2-1
YES
2
Sample Program
Address
Instruction
Comments
x3000
1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1
R1  x3100 (PC+0x0FF)
x3001
0 1 0 1 0 1 1 0 1 1 1 0 0 0 0 0
R3  0
x3002
0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0
R2  0
x3003
0 0 0 1 0 1 0 0 1 0 1 0 1 1 0 0
R2  12
x3004
0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1
If Z, goto x300A (PC+5)
x3005
0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0
Load next value to R4
x3006
0 0 0 1 0 1 1 0 1 1 0 0 0 1 0 0
Add to R3
x3007
0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1
Increment R1 (pointer)
x3008
0 0 0 1 0 1 0 0 1 0 1 1 1 1 1 1
Decrement R2 (counter)
x3009
0 0 0 0 1 1 1 1 1 1 1 1 1 0 1 0
Goto x3004 (PC-6)
3
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JMP (Register)
Jump is an unconditional branch -- always taken.
 Target address is the contents of a register.
 Allows any target address.
We could have used this as the last
instruction in the previous sample
program, but we would have had to put
the destination address into a register first.
4
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TRAP
Calls a service routine, identified by 8-bit “trap vector.”
vector routine
x23
input a character from the keyboard
x21
output a character to the monitor
x25
halt the program
When routine is done,
PC is set to the instruction following TRAP.
(We’ll talk about how this works later.)
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Another Example
Count the occurrences of a character in a file
 Program begins at location x3000
 Read character from keyboard
 Load each character from a “file”
• File is a sequence of memory locations
• Starting address of file is stored in the memory location
immediately after the program
 If file character equals input character, increment counter
 End of file is indicated by a special ASCII value: EOT (x04)
 At the end, print the number of characters and halt
(assume there will be less than 10 occurrences of the character)
A special character used to indicate the end of a sequence
is often called a sentinel.
 Useful when you don’t know ahead of time how many times
to execute a loop.
6
7
Program (1 of 2)
5-8
Address
Instruction
Comments
x3000
0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0
R2  0 (counter)
x3001
0 0 1 0 0 1 1 0 0 0 0 1 0 0 0 0
R3  M[x3102] (ptr)
x3002
1 1 1 1 0 0 0 0 0 0 1 0 0 0 1 1
Input to R0 (TRAP x23)
x3003
0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0
R1  M[R3]
x3004
0 0 0 1 1 0 0 0 0 1 1 1 1 1 0 0
R4  R1 – 4 (EOT)
x3005
0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0
If Z, goto x300E
x3006
1 0 0 1 0 0 1 0 0 1 1 1 1 1 1 1
R1  NOT R1
x3007
0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1
R1  R1 + 1
X3008
0 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0
R1  R1 + R0
x3009
0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1
If N or P, goto x300B
Program (2 of 2)
5-9
Address
Instruction
Comments
x300A
0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 1
R2  R2 + 1
x300B
0 0 0 1 0 1 1 0 1 1 1 0 0 0 0 1
R3  R3 + 1
x300C
0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0
R1  M[R3]
x300D
0 0 0 0 1 1 1 1 1 1 1 1 0 1 1 0
Goto x3004
x300E
0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0
R0  M[x3013]
x300F
0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0
R0  R0 + R2
x3010
1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 1
Print R0 (TRAP x21)
x3011
1 1 1 1 0 0 0 0 0 0 1 0 0 1 0 1
HALT (TRAP x25)
X3012
Starting Address of File
x3013
0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0
ASCII x30 (‘0’)
LC-3
Data Path
Revisited
Filled arrow
= info to be processed.
Unfilled arrow
= control signal.
10
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Data Path Components
Global bus
 special set of wires that carry a
16-bit signal to many components
 inputs to the bus are “tri-state
devices,” that only place a signal
on the bus when they are enabled
 only one (16-bit) signal should be
enabled at any time
• control unit decides which
signal “drives” the bus
 any number of components can
read the bus
• register only captures bus
data if it is write-enabled by
the control unit
Memory
 Control and data registers for
memory and I/O devices
 memory: MAR, MDR (also
control signal for read/write)
11
Data Path Components
ALU
 Accepts inputs from register file
and from sign-extended bits from IR (immediate field).
 Output goes to bus.
• used by condition code logic, register file, memory
Register File
 Two read addresses (SR1, SR2), one write address (DR)
 Input from bus
• result of ALU operation or memory read
 Two 16-bit outputs
• used by ALU, PC, memory address
• data for store instructions passes through ALU
5-12
Data Path Components
PC and PCMUX

Three inputs to PC, controlled by PCMUX
1. PC+1 – FETCH stage
2. Address adder – BR, JMP
3. bus – TRAP (discussed later)
MAR and MARMUX

5-13
Two inputs to MAR, controlled by MARMUX
1. Address adder – LD/ST, LDR/STR
2. Zero-extended IR[7:0] -- TRAP (discussed later)
Data Path Components
Condition Code Logic
 Looks at value on bus and generates N, Z, P signals
 Registers set only when control unit enables them (LD.CC)
• only certain instructions set the codes
(ADD, AND, NOT, LD, LDI, LDR, LEA)
Control Unit – Finite State Machine
 On each machine cycle, changes control signals for next phase
of instruction processing
• who drives the bus? (GatePC, GateALU, …)
• which registers are write enabled? (LD.IR, LD.REG, …)
• which operation should ALU perform? (ALUK)
•…
 Logic includes decoder for opcode, etc.
5-14
Computer Science 210 s1c
Computer Systems 1
2014 Semester 1
Lecture Notes
Lecture 11
Chapter 7: Assembly
Language
James Goodman (revised by Robert Sheehan)
Credits: Slides prepared by Gregory T. Byrd, North Carolina State University
15
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Human-Readable Machine Language
Computers like ones and zeros…
0001110010000110
Humans like symbols…
ADD R6,R2,R6
; increment index reg.
Assembler is a program that turns symbols into
machine instructions.
 ISA-specific:
close correspondence between symbols and instruction set
• mnemonics for opcodes
• labels for memory locations
 additional operations for allocating storage and initializing data
16
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LC-3 Assembly Language Syntax
Each line of a program is one of the following:
 an instruction
 an assembler directive (or pseudo-op)
 a comment
Whitespace (between symbols) and case are ignored.
Comments (beginning with “;”) are also ignored.
An instruction has the following format:
LABEL OPCODE OPERANDS ; COMMENTS
optional
mandatory
17
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Opcodes and Operands
Opcodes
 reserved symbols that correspond to LC-3 instructions
 listed in Appendix A
• ex: ADD, AND, LD, LDR, …
Operands





registers -- specified by Rn, where n is the register number
numbers -- indicated by # (decimal) or x (hex) or b (binary)
label -- symbolic name of memory location
separated by comma
number, order, and type correspond to instruction format
• ex:
ADD R1,R1,R3
ADD R1,R1,#3
LD R6,NUMBER
BRz LOOP
18
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Labels and Comments
Label
 placed at the beginning of the line
 assigns a symbolic name to the address corresponding to line
• ex:
LOOP ADD R1,R1,#-1
BRp LOOP
Comment




anything after a semicolon is a comment
ignored by assembler
used by humans to document/understand programs
tips for useful comments:
• avoid restating the obvious, as “decrement R1”
• provide additional insight, as in “accumulate product in R6”
• use comments to separate pieces of program
19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Assembler Directives
Pseudo-operations
 do not refer to operations executed by program
 used by assembler
 look like instruction, but “opcode” starts with a full stop
Opcode
Operand
Meaning
.ORIG
address
starting address of program
end of program
.END
.BLKW
n
allocate n words of storage
.FILL
n
allocate one word, initialize with value n
.STRINGZ
n-character
string
allocate n+1 locations, initialize
w/characters and null terminator
20
Hello World in LC-3 Assembler
; Hello world in LC-3 assembler
; I could have just used "PUTS" but that isn't fun.
.ORIG
LEA
loop
LDR
BRz
TRAP
ADD
BRnzp
finish TRAP
hello
x3000
R1, hello
R0, R1, #0
finish
x21
R1, R1, #1
loop
x25
; R1 points to next character
; R0 holds next character
; or just OUT prints R0[7:0]
; or HALT
.STRINGZ "Hello world"
.END
21
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Trap Codes
LC-3 assembler provides “pseudo-instructions” for
each trap code, so you don’t have to remember them.
Code
Equivalent
Description
HALT
TRAP x25
Halt execution and print message to console.
IN
TRAP x23
Print prompt on console,
read (and echo) one character from keybd.
Character stored in R0[7:0].
OUT
TRAP x21
Write one character (in R0[7:0]) to console.
GETC
TRAP x20
Read one character from keyboard.
Character stored in R0[7:0].
PUTS
TRAP x22
Write null-terminated string to console.
Address of string is in R0.
22
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Style Guidelines
Use the following style guidelines to improve
the readability and understandability of your programs:
1. Provide a program header, with author’s name, date, etc.,
and purpose of program.
2. Start labels, opcode, operands, and comments in same column
for each line. (Unless entire line is a comment.)
3. Use comments to explain what each register does.
4. Give explanatory comment for most instructions.
5. Use meaningful symbolic names.
• Mixed upper and lower case for readability.
• ASCIItoBinary, InputRoutine, SaveR1
6. Provide comments between program sections.
7. Each line must fit on the page -- no wraparound or truncations.
• Long statements split in aesthetically pleasing manner.
23
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Sample Program
Count the occurrences of a character in a file.
OUTPUT:
TEST:
BRz OUTPUT
24
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
ADD
R4, R1, #-4
; Test for EOT (ASCII x04)
BRz
OUTPUT
; If done, prepare the output
;
; Test character for match. If a match, increment count.
;
NOT
R1, R1
ADD
R1, R1, R0
; If match, R1 = xFFFF
NOT
R1, R1
; If match, R1 = x0000
BRnp
GETCHAR
; If no match, do not increment
ADD
R2, R2, #1
;
; Get next character from file.
;
GETCHAR
ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
25
Do this
Download the LC‐3 simulator package from the resources page
<http://www.cs.auckland.ac.nz/compsci210s1c/resources/>. For
running on Windows, read the document LC3WinGuide.pdf. (You may
run the simulator under Linux: read the document LC3_unix.pdf).
Follow the instructions for running a program, creating the files described
in the example and execute the program.
Create a source file from the text of program discussed in the lecture
(figures 5.16 & 7.2 in the book).
Create a “file” starting in the memory at location x4000.
Assemble the programme.
Execute the programme, typing different characters and make sure the
programme prints the correct result.
What goes wrong if the character you enter occurs more than 10 times in the
file?
26
Computer Science 210 s1c
Computer Systems 1
2014 Semester 1
Lecture Notes
Lecture 12
The Assembly Process
James Goodman (revised by Robert Sheehan)
Credits: Slides prepared by Gregory T. Byrd, North Carolina State University
27
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
First Pass: Constructing the Symbol Table
1. Find the .ORIG statement,
which tells us the address of the first instruction.
•
Initialize location counter (LC), which keeps track of the
current instruction.
2. For each non-empty line in the program:
a) If line contains a label, add label and LC to symbol table.
b) Increment LC.
– NOTE: If statement is .BLKW or .STRINGZ,
increment LC by the number of words allocated.
3. Stop when .END statement is reached.
NOTE: A line that contains only a comment is considered an empty line.
28
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
29
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
ADD
R4, R1, #-4
; Test for EOT (ASCII x04)
BRz
OUTPUT
; If done, prepare the output
;
; Test character for match. If a match, increment count.
;
NOT
R1, R1
ADD
R1, R1, R0
; If match, R1 = xFFFF
NOT
R1, R1
; If match, R1 = x0000
BRnp
GETCHAR
; If no match, do not increment
ADD
R2, R2, #1
;
; Get next character from file.
;
GETCHAR
ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
30
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
PTR
Address
?
31
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
BRz
OUTPUT
; If done, prepare the output
;
; Test character for match. If a match, increment count.
;
NOT
R1, R1
ADD
R1, R1, R0
; If match, R1 = xFFFF
NOT
R1, R1
; If match, R1 = x0000
BRnp
GETCHAR
; If no match, do not increment
ADD
R2, R2, #1
;
; Get next character from file.
;
GETCHAR
ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
32
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
33
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
NOT
R1, R1
ADD
R1, R1, R0
; If match, R1 = xFFFF
NOT
R1, R1
; If match, R1 = x0000
BRnp
GETCHAR
; If no match, do not increment
ADD
R2, R2, #1
;
; Get next character from file.
;
GETCHAR
ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
34
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
35
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
ADD
R2, R2, #1
;
; Get next character from file.
;
GETCHAR
ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
36
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
GETCHAR
?
37
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
LDR
R1, R3, #0
; R1 gets next char to test
BRnzp
TEST
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
38
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
GETCHAR
?0x300B
39
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
R1, R3, #0
; R1 gets next char to test
0x300C LDR
TEST
0x300D BRnzp
;
; Output the count.
;
OUTPUT
LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
40
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
GETCHAR
0x300B
41
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
GETCHAR
0x300B
42
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
R1, R3, #0
; R1 gets next char to test
0x300C LDR
TEST
0x300D BRnzp
;
; Output the count.
;
OUTPUT 0x300E LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
43
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
?
0x300E
GETCHAR
0x300B
44
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
R1, R3, #0
; R1 gets next char to test
0x300C LDR
TEST
0x300D BRnzp
;
; Output the count.
;
OUTPUT 0x300E LD
R0, ASCII
; Load the ASCII template
ADD
R0, R0, R2
; Convert binary count to ASCII
OUT
; ASCII code in R0 is displayed.
HALT
; Halt machine
;
; Storage for pointer and ASCII template
;
ASCII
.FILL
x0030
PTR
.FILL
x4000
.END
45
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
0x300E
GETCHAR
0x300B
ASCII
?
46
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
R1, R3, #0
; R1 gets next char to test
0x300C LDR
TEST
0x300D BRnzp
;
; Output the count.
;
OUTPUT 0x300E LD
R0, ASCII
; Load the ASCII template
ADD
R0,
R0,
R2
; Convert binary count to ASCII
0x300F
; ASCII code in R0 is displayed.
0x3010 OUT
; Halt machine
0x3011 HALT
;
; Storage for pointer and ASCII template
;
ASCII 0x3012 .FILL
x0030
PTR
.FILL
x4000
.END
47
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
TEST
0x3004
OUTPUT
0x300E
GETCHAR
0x300B
ASCII
?0x3012
48
;
;
;
;
;
;
;
;
Program to count occurrences of a character in a file.
Character to be input from the keyboard.
Result to be displayed on the monitor.
Program only works if no more than 9 occurrences are found.
Initialization
0x3000
0x3001
0x3002
0x3003
.ORIG
AND
LD
GETC
LDR
x3000
R2, R2, #0
R3, PTR
;
;
;
;
R2
R3
R0
R1
is counter, initially 0
is pointer to characters
gets character input
gets first character
R1, R3, #0
;
; Test character for end of file
;
TEST
R4, R1, #-4
; Test for EOT (ASCII x04)
0x3004 ADD
OUTPUT
; If done, prepare the output
0x3005 BRz
;
; Test character for match. If a match, increment count.
;
R1, R1
0x3006 NOT
R1, R1, R0
; If match, R1 = xFFFF
0x3007 ADD
R1, R1
; If match, R1 = x0000
0x3008 NOT
GETCHAR
; If no match, do not increment
0x3009 BRnp
R2, R2, #1
0x300A ADD
;
; Get next character from file.
;
GETCHAR 0x300B ADD
R3, R3, #1
; Point to next character.
R1, R3, #0
; R1 gets next char to test
0x300C LDR
TEST
0x300D BRnzp
;
; Output the count.
;
OUTPUT 0x300E LD
R0, ASCII
; Load the ASCII template
ADD
R0,
R0,
R2
; Convert binary count to ASCII
0x300F
; ASCII code in R0 is displayed.
0x3010 OUT
; Halt machine
0x3011 HALT
;
; Storage for pointer and ASCII template
;
ASCII 0x3012 .FILL
x0030
PTR
x4000
0x3013 .FILL
.END
49
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Symbol Table Construction
Construct the symbol table for the program in Figure 7.1 .
Symbol
Address
PTR
?
0x3013
TEST
0x3004
OUTPUT
0x300E
GETCHAR
0x300B
ASCII
0x3012
50
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Second Pass: Generating Machine Language
For each executable assembly language statement,
generate the corresponding machine language instruction.
 If operand is a label,
look up the address from the symbol table.
Potential problems:
 Improper number or type of arguments
• ex: NOT R1,#7
ADD R1,R2
ADD R3,R3,NUMBER
 Immediate argument too large
• ex: ADD R1,R2,#1023
 Address (associated with label) more than 256 from instruction
• can’t use PC-relative addressing mode
51
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Practice
Using the symbol table constructed earlier,
translate these statements into LC-3 machine language.
Statement
LD
R3,PTR
ADD
R4,R1,#-4
LDR
R1,R3,#0
Machine Language
BRnp GETCHAR
52
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Practice
Practice
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Using the symbol table constructed earlier,
translate
these table
statements
into LC-3
machine language.
Using
the symbol
constructed
earlier,
translate these statements into LC-3 machine language.
Statement
LD
R3,PTR
ADD
R4,R1,#-4
LDR
R1,R3,#0
BRnp GETCHAR
Machine Language
0010 0110 0001 0001
0001 1000 0111 1100
0110 0010 1100 0000
0000 1010 0000 0001
53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
LC-3 Assembler
Using “assemble” (Unix) or LC3Edit (Windows),
generates several different output files.
This one gets
loaded into the
simulator.
54
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Object File Format
LC-3 object file contains
 Starting address (location where program must be loaded),
followed by…
 Machine instructions
Example
 Beginning of “count character” object file looks like this:
.ORIG x3000
0011000000000000
AND R2, R2, #0
0101010010100000
LD R3, PTR
0010011000010001
1111000000100011
TRAP x23
.
.
.
55
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Multiple Object Files
An object file is not necessarily a complete program.
 system-provided library routines
 code blocks written by multiple developers
For LC-3 simulator,
can load multiple object files into memory,
then start executing at a desired address.
 system routines, such as keyboard input, are loaded automatically
• loaded into “system memory,” below x3000
• user code should be loaded between x3000 and xFDFF
 each object file includes a starting address
 be careful not to load overlapping object files
56
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Linking and Loading
Loading is the process of copying an executable image
into memory.
 more sophisticated loaders are able to relocate images
to fit into available memory
 must readjust branch targets, load/store addresses
Linking is the process of resolving symbols between
independent object files.
 suppose we define a symbol in one module,
and want to use it in another
 some notation, such as .EXTERNAL, is used to tell assembler that a
symbol is defined in another module
 linker will search symbol tables of other modules to resolve symbols
and complete code generation before loading
57
News from the NYTimes (18 June 1996)
“When a computer runs out of [main memory],
modern operating systems automatically use the
memory on the hard drive. But today’s hard drives
retrieve data at speeds of about 10 milliseconds
(millionths of a second). That seems fast until you
consider that modern RAM can do this at 60
nanoseconds (billionths of a second), more than 150
times as fast.”
What’s wrong with this statement??
58
“Correction: June 19, 1996, Wednesday The Personal Computers
column in Science Times yesterday, about options for increasing
computer memory, incorrectly described milliseconds (they are
thousandths of a second, not millionths) and therefore misstated the
difference in speed between random access memory (RAM) and hard
drives. RAM can be more than 100,000 times, not 150 times, as fast.”
59
Time Line
1 second
10-10
10-9
1 minute
10-8
10-7
1 hour
10-6
10-5
1 day
10-4
1 month
10-3
10-2
1 year
10-1
100
Time (Logarithmic Scale)
Scale by 31,557,600
60
Speed Line
Execute
one
instruction
(best case)
One clock period
2 GHz
Cache miss time
(Memory access time)
Transfer 1 char
at 56K baud
Cache hit time
Read 1 byte
from disk
10-10
10-9
10-8
10-7
10-6
10-5
10-4
One disk revolution
(6-8 ms)
Total Disk
access time
10-3
10-2
10-1
100
Time (Logarithmic Scale)
Time for light to travel 30 cm
Time for sound to travel 30 cm
61