Download Lecture 4

1/4/2013
Introduction to Programming in
Assembler Language
Lecture 4
What Does a Microprocessor Know?
A microprocessor “understands” only binary
information; numbers and characters are just
patterns of 0’s
0 s and 1
1’s;
s; program instructions are
just patterns of 0’s and 1’s
SUBQ.L #1,D2
=> 0x5382 = 0b0101001110000010
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Machine Language
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Not all possible binary patterns may correspond to legal
CPU instructions
For the Coldfire architecture, instructions occupy
anywhere from 1 to 3 subsequent words in memory,
- where each word has 16 bits
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Machine Language (Cont’d)
For a MOVE instruction: move.l #0x20000, d1
movement Byte: 01 Reg #: 001 Data reg:
Word: 11
000
Long:10
Immediate
data: 111
100
.org 0x1000
move.l #0x20000,d1
.end
=>
0x1000 22 3c 0x1001
0x1002 00 02 0x1003
0x1004 00 00 0x1005
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Machine Language (Cont’d)
bits 15-14: 00, which means a data movement
bits 13-12: size. Since the size is longword, here bits 13-12 are 10
bits 11-6
bi
11 6 are for
f d
destination,
i i
where
h
bits
bi 8-6
8 6 are to specify
if the
h "mode"
" d " off
the destination (i.e., is the destination a data register, an address register,
or memory location?). Since here the destination is a data register D1,
the three destination mode bits are 000, which are corresponding to data
register. Bits 11-9 are destination register number, which are used to
specify "which" data register is the destination. Since here we have data
register #1, bits 11-9 are 001, which is the binary code for number 1.
bits 5-0 are for source
source, where bits 5-3 are mode
mode, and bits 2-0 are register
number. If the operand is an immediate number, the mode bits are always
111, and the register number bits are always 100.
Therefore, the operation word for the example data movement instruction
is: 0010 001000 111100 in binary, which is 0x22 3C.
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Machine Language (Cont’d)
In addition, the six bits for the source (i.e., bits 5-0) only tell that the
source is an immediate number. But they do not tell "what" value it is.
Therefore, we need extension words to tell what value the immediate
number
b is.
i Since
Si
the
h operation
i value
l is
i a long
l
word
d (as
( specified
ifi d by
b .L),
L)
we write the source value in a longword format, which is 0x 0002 0000.
Then, we need two extension words to specify what the longword-sized
immediate number is. Since the first word is the most significant word,
the first extension word is 0x 0002, while the second extension words is
0x 0000.
In a summary, we need three words for the example data movement
instruction which are 0x 223C 0002 0000.
instruction,
0000
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Assembly Language
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Assembly Language is a symbolic, more “human
friendly” notation for expressing programs made
up of machine language instructions;
move.b, move.w, clr.l
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Instruction mnemonics, decimal numbers,
alphanumeric labels can all be used
start add #1, D0
move.l A, D0
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Assembly vs. Machine Languages
Address Contents
8000
8002
8004
8006
8008
800A
800C
800E
8010
7200
227C
0002
0000
7404
D299
5382
66FA
4E4F
INIT
LOOP
MOVEQ.L #0,D1
MOVEA.L #0x20000,A1
; second word
; third word
MOVEQ.L
#4,D2
ADD.L
(A1)+,D1
SUBQ.L
#1,D2
BNE
LOOP
TRAP
#15
INIT is the name for address location 0x8000
LOOP is the name for address 0x800A
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Assembly Language
A mnemonic is an aid to memory
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“clear a byte of data register D5” in binary is
0b 0100 0010 0000 0101
or in hexadecimal 0x 4205
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The mnemonic representation is:
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0b: binary
0x: hex
CLR.B D5
much more easy to understand
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Addresses and constants can be written in a symbolic form
A valid symbolic name is created by the programme, e.g. Temp1,
A, B, Var_01
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Assembly Language Syntax
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create an assembly language source file using any text editor
vi, kwrite, emacs, winedit, notepad, etc.
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An assembly language instruction is one of two types:
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an executable instruction:
a valid processor instruction, e.g.
MOVE.B D0, D7
ADD.L D5, A7
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an assembler directive: it is not a processor instruction,
is not translated into machine code (binary),
1) link symbolic names to actual values, e.g., .equ TEN, 10
2) allocate storage for data in memory, e.g., ds.w 4
3) set up a space for pre-defined constants, e.g., .byte 23
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Assembly Language Syntax
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An assembly language instruction has up to four fields:
label
opcode operands comment
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label is used to reference this instruction; it also becomes
the name of the address where the instruction is stored;
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opcode is the machine operation code
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E.g.
ADD, add, MOVE, CLR
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operands indicate the locations of source data and result of
the operation
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comment is a comment to assist the reader in understanding
what the instruction is about, indicated by /*…*/
/* This is the beginning of the main algorithm */
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Assembly Language Syntax
EXAMPLE
.org
0x001000 /*program to start at 0x1000 */
/* this is an example */
start
move.l d0, d1 /*this instruction copies the*/
/*contents of d0 into d1 */
/*the RTL notation is [d1] ←[d0] */
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Basic Concepts
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Assembling
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Is the process of translating an assembly language
program into a machine language program;
“hand assembling” is when a human assembles the
program without using any program aids.
Assembler
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Is a program that automates the process of translating an
assembly language program into the corresponding
machine language program
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Basic Concepts
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Disassembling
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Lecture 4
Is the reverse p
process to assembling
g in which an
assembly language program is reconstructed from a
machine language program; disassembling
Can be useful for debugging and reverse engineering
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Basic Concepts
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If the assembler runs on the same machine that
runs the machine code it is called a resident
assembler
If the assembler runs on another machine it is
called a cross – assembler
In this situation the code must be transferred from
the host computer (where the assembler runs) to
the target machine (where the object code runs).
This process is called down loading
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Basic Concepts
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Lecture 4
The process of moving object code into memory
and making it ready to run is called loading
Some assemblers permit programs to be broken
into several files which can be assembled
independently; a program which combines several
object files into one executable file is called a
linker
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