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ASSEMBLY LANGUAGE
&ASSEMBLER DESIGN
Part-2
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Human-Readable Machine Language
Computers like ones and zeros…
Humans like symbols…
Assembler is a program that turns symbols intomachine instructions.
• ISA-specific:close correspondence between symbols and instruction set
mnemonics for opcodeslabels for memory locations
• additional operations for allocating storage and initializing data
ADD R6,R2,R6 ; increment index reg.
0001110010000110
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LC-3 Assembly Language Syntax
Each line of a program is one of the following:• an instruction• an assember 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
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Opcodes and Operands
Opcodes• reserved symbols that correspond to LC-3 instructions.ex: ADD , AND , LD, LDR , …
Operands• registers -- specified by Rn, where n is the register number• numbers -- indicated by # (decimal) or x (hex)• label -- symbolic name of memory location• number, order, and type correspond to instruction format
ex: ADD R1,R1,R3 ADD R1,R1,#3
LD R6,NUMBER BRz LOOP
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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
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Assembler Directives
Pseudo-operations• do not refer to operations executed by program• used by assembler• look like instruction, but “ opcode ” starts with dot
Opcode Operand Meaning .ORIG address starting address of program.END end of program.BLKW n allocate n words of storage
.FILL n allocate one word, initialize withvalue n
.STRINGZ n-characterstring
allocate n+1 locations,initialize w/characters and nullterminator
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An Assembly Language Program;
; Program to multiply a number by the constant 6;.ORIG x3050LD R1, SIXLD R2, NUMBER
AND R3, R3, #0 ; Clear R3. It will; contain the product.
; The inner loop
AGAIN ADD R3, R3, R2 ADD R1, R1, #-1 ; R1 keeps track of
BRp AGAIN ; the iteration.
HALT NUMBER .FILL #20
SIX .FILL x0006
.END
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; Program to multiply a number by the constant 6.ORIG x3050LD R1, SIX ; R1 is loop counterLD R2, NUMBER
AND R3, R3, #0 ; Clr R3, will hold product; The Loop
AGAIN ADD R3, R3, R2 ; Summing into R3
ADD R1, R1, #-1 ; Dec loop counterBRp AGAINHALT
; NUMBER .FILL #20
SIX .FILL x0006.END
Must have Opcode and Operands
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; Program to multiply a number by the constant 6.ORIG x3050LD R1, SIX ; R1 is loop counterLD R2, NUMBER
AND R3, R3, #0 ; Clr R3, will hold product; The Loop
AGAIN ADD R3, R3, R2 ; Summing into R3
ADD R1, R1, #-1 ; Dec loop counterBRp AGAINHALT
; NUMBER .FILL #20
SIX .FILL x0006.END
Label and Comments optional
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; Program to multiply a number by the constant 6.ORIG x3050LD R1, SIX ; R1 is loop counterLD R2, NUMBER
AND R3, R3, #0 ; Clr R3, will hold product; The Loop
AGAIN ADD R3, R3, R2 ; Summing into R3
ADD R1, R1, #-1BRp AGAINHALT
NUMBER .FILL #20SIX .FILL x0006
.END
Decimal #Binary bHex x
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; Program to multiply a number by the constant 6.ORIG x3050LD R1, SIX ; R1 is loop counterLD R2, NUMBER
AND R3, R3, #0 ; Clr R3, will hold product; The Loop
AGAIN ADD R3, R3, R2 ; Summing into R3
ADD R1, R1, #-1BRp AGAINHALT
NUMBER .FILL #20SIX .FILL x0006
.END
More than just code, need to
worry about memory allocation
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Assembler
• Translating source code written in assembly language to objectcode.
• Assigning machine address to symbolic labels.
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Assembler-Design
• Assembler Design can be done in: – Single pass – Two pass
• Single Pass Assembler: – Does everything in single pass – Cannot resolve the forward referencing
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Assembler- Design…•
Symbol Table: – This is created during pass 1 – All the labels of the instructions are symbols – Tables has entry for symbol name, address value.
• Forward reference: – Symbols that are defined in the later part of the
program are called forward referencing. – There will not be any address value for such symbols
in the symbol table in pass 1.
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Assembly Process
Convert assembly language file (.asm)into an executable file (.obj) for the LC-3 Assembler.
First Pass:• Scan program file• Find all labels and calculate the corresponding addresses;
this is called the symbol table
Second Pass:• Convert instructions to machine language , using information from
symbol table
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First Pass: Constructing the Symbol Table
1. Find the .ORIG statement , which tells us the addressof 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.
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PracticeConstruct the symbol table for the program
Symbol AddressSIX
NUMBER
AGAIN
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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 argumentsex: NOT R1,#7
ADD R1,R2ADD R3,R3,NUMBER
• Immediate argument too largeex: ADD R1,R2,#102 3
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Practice
Using the symbol table constructed earlier,translate these statements into LC-3 machine language .
Statement Machine LanguageLD R3,PTR
ADD R4,R1,#-4
LDR R1,R3,#0
BRnp GETCHAR
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Example-2 ,Program with object code
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Assembler algorithm and data structures
• OPTAB: operation code table• SYMTAB: symbol table• LOCCTR: location counter
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Algorithm Pass 1
Beginread first input lineif OPCODE = ‘START’ then begin
save #[Operand] as starting addr
initialize LOCCTR to starting addresswrite line to intermediate fileread next line
end ( if START)else
initialize LOCCTR to 0
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Algorithm Pass 1…While OPCODE != ‘END’ do
beginif this is not a comment line thenbeg
if there is a symbol in the LABEL field thenbegin
search SYMTAB for LABELif found thenset error flag (duplicate symbol)
else(if symbol)
search OPTAB for OPCODEif found then
add 3 (instr length) to LOCCTRelse if OPCODE = ‘WORD’ then
add 3 to LOCCTR
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Algorithm Pass 1…
else if OPCODE = ‘RESW’ thenadd 3 * #[OPERAND] to LOCCTR
else if OPCODE = ‘RESB’ thenadd #[OPERAND] to LOCCTR
else if OPCODE = ‘BYTE’ thenbegin
find length of constant in bytesadd length to LOCCTR
endelse
set error flag (invalid operation code)end (if not a comment)
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Algorithm Pass 1…
write line to intermediate fileread next input line
end { while not END}write last line to intermediate file
Save (LOCCTR – starting address) as program lengthEnd {pass 1}
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Pass 2 Algorithm
Beginread 1 st input line
if OPCODE = ‘START’ thenbegin
write listing lineread next input line
endwrite Header record to object programinitialize 1 st Text record
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Pass 2 Algorithm…
while OPCODE != ‘END’ dobegin
if this is not comment line thenbegin
search OPTAB for OPCODEif found then
beginif there is a symbol in OPERAND field then
begin
search SYMTAB for OPERAND field thenif found then
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Pass 2 Algorithm…begin
store symbol value as operand addresselsebegin
store 0 as operand addressset error flag (undefined symbol)
endend (if symbol)
else store 0 as operand addressassemble the object code instruction
else if OPCODE = ‘BYTE’ or ‘WORD” thenconvert constant to object code
if object code doesn’t fit into current Text record thenbegin
Write text record to object codeinitialize new Text record
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Pass 2 Algorithm…
endadd object code to Text record
end {if not comment}write listing line
read next input lineend
write listing lineread next input linewrite last listing line
End {Pass 2}
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Converting Code to Assembly
Can use a standard template approach
Typical Constructs:
• if/else• while• do/while• for
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if/else
if(x > 0){
r2 = r3 + r4;
}else
{
r5 = r6 + r7;}
LD R1, XBRP THEN
ADD R5,R6,R7
BRNZP DONETHEN ADD R2,R3,R4
DONE ...
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if/else
if(x > 0){
r2 = r3 + r4;
}else
{
r5 = r6 + r7;}
LD R1, XBRNZ ELSE
ADD R2,R3,R4
BRNZP DONEELSE ADD R2,R3,R4
DONE ...
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while
i = 10;x = 0;while(i > 0)
{x = x + i;i--;
}
AND R1,R1,#0 ADD R1,R1,#10
AND R2,R2,#0
WHL BRNZ DONE ADD R2,R2,R1
ADD R1,R1,#-1
BRNZP WHL
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LC-3 Assembler
Using “assembler” (Unix) or LC3Edit (Windows),generates several different output files.
This one gets loaded into the simulator.
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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:
00110000000000000101010010100000
00100110000100011111000000100011
.
.
.
.ORIG x3000
AND R2, R2, #0
LD R3, PTR TRAP x23
h h ll d f d d l
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Role of Loader
Source
Program TranslatorObject
Program
LoaderObject
programready forexecution
Memory
Translator – Assembler/Compiler
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Role of Loader
Source
Program AssemblerObject
Program
LoaderObject
programready forexecution
Memory
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Role of Loader and Linker
Source
Program AssemblerObject
Program
Linker
ExecutableCode
Loader
Objectprogramready forexecution
Memory
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Linking and Loading
Linking is the process of resolving symbols betweenindependent 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 asymbol is defined in another module• linker will search symbol tables of other modules to resolve symbols
and complete code generation before loading
Loading is the process of copying an executable imageinto memory.
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