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What is Assembly Language?
Introduction to the GNU/Linux
assembler and linker
for Intel Pentium processors
High-Level Language
• Most programming nowdays is done using
so-called “high-level” languages (such as
FORTRAN, BASIC, COBOL, PASCAL, C,
C++, JAVA, SCHEME, Lisp, ADA, etc.)
• These languages deliberately “hide” from
a programmer many details concerning
HOW his problem actually will be solved
by the underlying computing machinery
The BASIC language
• Some languages allow programmers to
forget about the computer completely!
• The language can express a computing
problem with a few words of English, plus
formulas familiar from high-school algebra
• EXAMPLE PROBLEM: Compute 4 plus 5
The example in BASIC
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4
5
LET X = 4
LET Y = 5
LET Z = X + Y
PRINT X, “+”, Y, “=“, Z
END
Output:
4+5=9
The Python language
• It shares some of the best features of the
earlier BASIC language – but improves it
by incorporating more modern capabilities
• The Python Language Reference manual
is handily accessible online:
<http://docs.python.org/reference/>
• Lots of tutorials and program examples
can be quickly discovered using ‘google’
Example in Python
x=4
y=5
z=x+y
print x, ‘+’, y, ‘=‘, z
• If you type these Python statements into a
textfile named ‘example’, you can execute
them in a Windows or Linux environment:
– e.g.,
$ python example
The C language
• Other high-level languages do require a small
amount of awareness by the program-author of
how a computation is going to be processed
• For example, that:
- the main program will get “linked” with a
“library” of other special-purpose subroutines
- instructions and data will get placed into
separate sections of the machine’s memory
- variables and constants get treated differently
- data items have specific space requirements
Same example: rewritten in C
#include <stdio.h>
// needed for printf()
int
int
// initialized variables
// unitialized variable
x = 4, y = 5;
z;
int main()
{
z = x + y;
printf( “%d + %d = %d \n”, x, y, z );
}
“ends” versus “means”
• Key point: high-level languages let programmers
focus attention on the problem to be solved, and
not exspend effort thinking about details of “how”
a particular piece of electrical machiney is going
to carry out the pieces of a desired computation
• Key benefit: their problem gets solved sooner
(because their program can be written faster)
• Programmers don’t have to know very much
about how a digital computer actually works
computer scientist vs. programmer
• But computer scientists DO want to know
how computers actually work:
-- so we can diagnose computer errors
-- so we can employ optimum algorithms
-- so we can predict computer behavior
-- so we can devise faster computers
-- so we can build cheaper computers
-- so we can pick one suited to a problem
A machine’s own language
• For understanding how computers work,
we need familiarity with the computer’s
own language (called “machine language”)
• It’s LOW-LEVEL language (very detailed)
• It is specific to a machine’s “architecture”
• It is a language “spoken” using voltages
• Humans represent it with zeros and ones
Example of machine-language
Here’s what a program-fragment looks like:
10100001 10111100 10010011 00000100
00001000 00000011 00000101 11000000
10010011 00000100 00001000 10100011
11000000 10010100 00000100 00001000
It means:
z = x + y;
Incomprehensible?
• Though possible, it is extremely difficult, tedious
(and error-prone) for humans to read and write
“raw” machine-language
• When unavoidable, a special notation can help
(called hexadecimal representation):
A1 BC 93 04 08
03 05 C0 93 04 08
A3 C0 94 04 08
• But still this looks rather meaningless!
Hence assembly language
• There are two key ideas:
-- mnemonic opcodes: we use abbreviations of
English language words to denote operations
-- symbolic addresses: we invent “meaningful”
names for memory storage locations we need
• These make machine-language understandable
to humans – if they know their machine’s design
• Let’s see our example-program, rewritten using
actual “assembly language” for Intel’s Pentium
Simplified Block Diagram
Central
Processing
Unit
Main
Memory
system bus
I/O
device
I/O
device
I/O
device
I/O
device
The visible x86 registers
RAX
RSP
R8
R12
RBX
RBP
R9
R13
RCX
RSI
R10
R14
RDX
RDI
R11
R15
CS
DS
RIP
ES
FS
GS
RFLAGS
Intel Core-2 Quad processor
SS
The “Fetch-Execute” Cycle
main memory
central processor
Temporary
Storage
(STACK)
RSP
Program
Variables
(DATA)
Program
Instructions
(TEXT)
EAX
EAX
EAX
RAX
RIP
the system bus
Define symbolic constants
.equ
.equ
.equ
device_id, 1
sys_write, 1
sys_exit, 60
our program’s ‘data’ section
.section
x: .quad
y: .quad
fmt: .asciz
.data
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“%d + %d = %d \n”
Our program’s ‘bss’ section
.section
z: .quad
n: .quad
buf: .space
.bss
0
0
80
our program’s ‘text’ section
.section
.text
_start:
# comment: assign z = x + y
movl
x, %rax
addl
y, %rax
movl
%rax, z
‘text’ section (continued)
# comment: prepare program’s output
push
z
# arg 5
push
y
# arg 4
push
x
# arg 3
push
$fmt
# arg 2
push
$buf
# arg 1
call
sprintf
# function-call
addl
$40, %rsp # discard the args
movl
%rax, n
# save return-value
‘text’ section (continued)
# comment: request kernel assistance
movl
$sys_write, %rax
movl
$device_id, %rdi
movl
$buf, %rsi
movl
n, %rdx
syscall
‘text’ section (concluded)
# comment: request kernel assistance
movl
$sys_exit, %rax
movl
$0, %rdi
syscall
# comment: make label visible to linker
.global
_start
.end
program translation steps
demo.s
demo.o
demo
program
source
module
assembly
program
object
module
object module library
object module library
other object modules
linking
the
executable
program
The GNU Assembler and Linker
• With Linux you get free software tools for
compiling your own computer programs
• An assembler (named ‘as’): it translates
assembly language (called the ‘source code’)
into machine language (called the ‘object code’)
$ as demo.s -o demo.o
• A linker (named ‘ld’): it combines ‘object’ files
with function libraries (if you know which ones)
How a program is loaded
0xFFFFFFFFFFFFFFFF
Kernel’s code and data
stack
runtime libraries
.bss
.data
uninitialized variables
initialized variables
.text
program instructions
Main memory
0x0000000000000000
What must programmer know?
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Needs to use CPU register-names (rax)
Needs to know space requirements (quad)
Needs to know how stack works (push)
Needs to make symbol ‘global’ (for linker)
Needs to understand how to quit (exit)
And of course how to use system tools:
(e.g., text-editor, assembler, and linker)
Summary
• High-level programming (offers easy and
speedy real-world problem-solving)
• Low-level programming (offers knowledge
and power in utilizing machine capabilities)
• High-level language hides lots of details
• Low-level language reveals the workings
• High-level programs: are readily ‘portable’
• Low-level programs: tied to specific CPU
In-class exercise #1
• Download the source-file for ‘demo1’, and
compile it using the GNU C compiler ‘gcc’:
$ gcc demo1.c -o demo1
Website: http://cs.usfca.edu/~cruse/cs210/
• Execute this compiled application using:
$ ./demo1
In-class exercise #2
• Download the two source-files needed for our
‘demo2’ application (i.e., ‘demo2.s’ and
‘sprintf.s’), and assemble them using:
$ as demo2.s -o demo2.o
$ as sprintf.s -o sprintf.o
• Link them using:
$ ld demo2.o sprintf.o -o demo2
• And execute this application using: $ ./demo2
In-class exercise #3
• Use your favorite text-editor (e.g., ‘vi’) to
modify the ‘demo2.s’ source-file, by using
different initialization-values for x and y
• Reassemble your modified ‘demo2.s’ file,
and re-link it with the ‘sprintf.o’ object-file
• Run the modified ‘demo2’ application, and
see if it prints out a result that is correct
In-class exercise #4
• Download the ‘ljpages.cpp’ system-utility
from our class website and compile it:
$ g++ ljpages.cpp –o ljpages
• Execute this utility-program to print your
modified assembly language source-file:
$ ./ljpages demo2.s
• Write your name on the printed hardcopy
and turn it in to your course instructor
Summary of the exercises
Download and Compile a high-level program
Assemble and Link a low-level program
Edit and recompile an assembly program
Print out and turn in your hardcopy