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IA32 programming for Linux
Concepts and requirements for writing Linux assembly language
programs for Pentium CPUs
A source program’s format
• Source-file: a pure ASCII-character textfile
• Is created using a text-editor (such as ‘vi’)
• You cannot use a ‘word processor’ (why?)
• Program consists of series of ‘statements’
• Each program-statement fits on one line
• Program-statements all have same layout
• Design in 1950s was for IBM punch-cards
Statement Layout (1950s)
• Each ‘statement’ was comprised of four ‘fields’• Fields appear in a prescribed left-to-right order• These four fields were named (in order):
-- the ‘label’ field-- the ‘opcode’ field-- the ‘operand’ field-- the ‘comment’ field
• In many cases some fields could be left blank• Extreme case (very useful): whole line is blank!
The ‘as’ program
• The ‘assembler’ is a computer program
• It accepts a specified text-file as its input
• It must be able to ‘parse’ each statement
• It can produce onscreen ‘error messages’
• It can generate an ELF-format output file
• (That file is known as an ‘object module’)
• It can also generate a ‘listing file’ (optional)
The ‘label’ field
• A label is a ‘symbol’ followed by a colon (‘:’)• The programmer invents his own ‘symbols’• Symbols can use letters and digits, plus a very
small number of ‘special’ characters ( ‘.’, ‘_’, ‘$’ )• A ‘symbol’ is allowed to be of arbitrarily length • The Linux assembler (‘as’) was designed for
translating source-text produced by a high-level language compiler (such as ‘cc’)
• But humans can also write such files directly
The ‘opcode’ field
• Opcodes are predefined symbols that are recognized by the GNU assembler
• There are two categories of ‘opcodes’ (called ‘instructions’ and ‘directives’)
• ‘Instructions’ represent operations that the CPU is able to perform (e.g., ‘add’, ‘inc’)
• ‘Directives’ are commands that guide the work of the assembler (e.g., ‘.globl’, ‘.int’)
Instructions vs Directives
• Each ‘instruction’ gets translated by ‘as’ into a machine-language statement that will be fetched and executed by the CPU when the program runs (i.e., at ‘runtime’)
• Each ‘directive’ modifies the behavior of the assembler (i.e., at ‘assembly time’)
• With GNU assembly language, they are easy to distinguish: directives begin with ‘.’
A list of the Pentium opcodes
• An ‘official’ list of the instruction codes can be found in Intel’s programmer manuals:
http://developer.intel.com
• But it’s three volumes, nearly 1000 pages (it describes ‘everything’ about Pentiums)
• An ‘unofficial’ list of (most) Intel instruction codes can fit on one sheet, front and back:
http://www.jegerlehner/intel/
The AT&T syntax
• The GNU assembler uses AT&T syntax (instead of official Intel/Microsoft syntax) so the opcode names differ slightly from names that you will see on those lists:
Intel-syntax AT&T-syntax--------------- ---------------------- ADD addb/addw/addl INC incb/incw/incl CMP
cmpb/cmpw/cmpl
The UNIX culture
• Linux is intended to be a version of UNIX (so that UNIX-trained users already know Linux)
• UNIX was developed at AT&T (in early 1970s) and AT&T’s computers were built by DEC, thus UNIX users learned DEC’s assembley language
• Intel was early ally of DEC’s competitor, IBM, which deliberately used ‘incompatible’ designs
• Also: an ‘East Coast’ versus ‘West Coast’ thing (California, versus New York and New Jersey)
Bytes, Words, Longwords
• CPU Instructions usually operate on data-items• Only certain sizes of data are supported:
BYTE: one byte consists of 8 bits
WORD: consists of two bytes (16 bits)
LONGWORD: uses four bytes (32 bits)• With AT&T’s syntax, an instruction’s name also
incorporates its effective data-size (as a suffix) • With Intel syntax, data-size usually isn’t explicit,
but is inferred by context (i.e., from operands)
The ‘operand’ field
• Operands can be of several types:
-- a CPU register may hold the datum
-- a memory location may hold the datum
-- an instruction can have ‘built-in’ data
-- frequently there are multiple data-items
-- and sometimes there are no data-items• An instruction’s operands usually are ‘explicit’,
but in a few cases they also could be ‘implicit’
Examples of operands
• Some instruction that have two operands:movl %ebx, %ecxaddl $4, %esp
• Some instructions that have one operand:incl %eaxpushl $fmt
• An instruction that lacks explicit operands:ret
The ‘comment’ field
• An assembly language program often can be hard for a human being to understand
• Even a program’s author may not be able to recall his programming idea after awhile
• So programmer ‘comments’ can be vital
• A comments begin with the ‘#’ character
• The assembler disregards all comments (but they will appear in program listings)
‘Directives’
• Sometimes called ‘pseudo-instructions’
• They tell the assembler what to do
• The assembler will recognize them
• Their names begin with a dot (‘.’)
• Examples: ‘.section’, ‘.global’, ‘.int,’ …
• The names of valid directives appears in the table-of-contents of the GNU manual
New program example
• Let’s look at a demo program (‘squares.s’)• It prints out a mathematical table showing some
numbers and their squares• But it doesn’t use any multiplications!• It uses an algorithm based on algebra:
(n+1)2 - n2 = n + n + 1
If you already know the square of a given number n , you can get the square of the
next number n+1 by just doing additions
Visualizing the algorithm idean
n
(n + 1)2 = n2 + 2n + 1
A program with a ‘loop’
• Here’s our program idea (expressed in C)int num = 1, val = 1;do {
printf( “ %d %d \n”, num, val );val += num + num + 1;num += 1;}
while ( num <= 20 );
Some new ‘directives’
• ‘.equ’ – equates a symbol to a value:.equ MAX, 20
• ‘.globl’ – just an alternative for ‘.global’:.globl main
Some new ‘instructions’
• ‘inc’ – adds one to the specified operand:incl arg
• ‘cmp’ – compares two specified operands:cmpl $max, arg
• ‘jle’ – jump (to a specified instruction) if condition ‘less than or equal to’ is true:
jle again
Comparisons can be ‘tricky’
• It’s easy to get confused by AT&T syntax:
mov $5, %eax
while: inc %eax
cmp $5, %eax
jle while
(e.g., will this loop ever finish executing?)
• REMEMBER: ‘compare’ means ‘subtract’
The FLAGS register
OF
DF
IF
TF
SF
ZF
0AF
0PF
1CF
Legend: ZF = Zero FlagSF = Sign FlagCF = Carry FlagPF = Parity FlagOF = Overflow FlagAF = Auxiliary Flag
In-class exercise #1
• How would you modify the source-code for the ‘squares’ program so that it prints out a larger table (i.e., more than 20 lines)?
• How many squares can you display on the screen before your program starts to show ‘wrong’ entries?
In-class exercise #2
• Can you write a program that prints out a table showing powers of 2 (it’s useful for computer science students to keep handy)
• Can you see how to do it without using any ‘multiply’ operations – just additions?
• Hint: study the ‘squares.s’ source-code
• Then write your own ‘powers.s’ solution
• Turn in printouts (source and its output)
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