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Assembly Language / Development Process
Problem: must convert ideas (human thoughts) into an
executing program (binary image in memory)
Need:
DEVELOPMENT PROCESS
 people-friendly way to write programs
 tools to support conversion to binary image
 assembly language: used by people to describe
programs
• syntax: set of symbols + grammar rules for
constructing statements using symbols
• semantics: what is meant by statements; ultimately:
the binary image
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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Assembly Language / Development Process
assembler: program – converts programs from
assembly language to object format
• object format: an intermediate format
– mostly binary, but may include other info
linker: program that combines object files to
create an “executable” file
loader: loads executable files into memory, and
may initialize some registers (e.g. IP )
Assembler, linker and loader are tools.
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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Development Process
Editor
people
work
here
.LST
.ASM
.OBJ
.OBJ
Assembler
Linker
loader is part of
operating system
(or possibly
debugger)
simulator loads
.OBJ files directly
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human readable
results (including
assembly errors)
may link multiple
OBJ files
.EXE
Loader
memory
processor IP
94.201 - Fall 2001: copyright ©T. Pearce, D.Computer
Hutchinson,System
L. Marshall Sept. 2001
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p86 Assembly Language
 people create .ASM files using assembly
language
 syntax must account for all aspects of a
program and development process:
• constant values
• reserve memory to use for variables
• write instructions: operations & operands
– addressing modes
• directives to tools in development process
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Constants
binary value: consists of only 0’s and 1’s
 ends with ‘B’ or ‘b’, e.g. 10101110b
hexadecimal value: starts with 0 .. 9
 may include 0 . . 9, A .. F (a . . f )
 ends with ‘H’ or ‘h’, e.g. 0FFH (8-bit hex value)
decimal value: default format – no “qualifier” extension
 consists of digits in 0 . . 9, e.g. 12345
string: sequence of characters encoded as 7-bit ASCII
bytes:
 enclosed in single quotes, e.g. ‘Hi Mom’ (6 bytes)
 character: string with length = 1
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Labels
 user-defined names – represent addresses
 lets programmer refer to addresses using logical names
– no need for concern with exact hexadecimal values
 leave assembler to:
• decide exact addresses to use
• deal with hexadecimal addresses
 labels are used to identify addresses for:
• control flow – identify address of target
• memory variables – identify address where data is
stored
 labels serve in 2 roles:
• label definition and label reference
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Label Definition





used by assembler to decide exact address
must be first non-blank text on a line
user-defined name followed by “:”
name must start with alpha A .. Z a .. z
then contain: alpha, numeric, ‘_’
• e.g. Continue:
L8R:
Out_2_Lunch:
 cannot redefine reserved words
• e.g. MOV: is illegal
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p86 Assembly Language: Label
Address/Reference
 label represents address of first allocated byte after definition
• e.g. DoThis: MOV AX, [ BX ]
• DoThis represents address of first byte of the MOV
instruction
 label reference: use of label in an operand
• refers to address assigned by assembler (no “:”)
 control flow example (assume CX contains loop counter):
DoWhile:
CMP
CX, 0
JE
DoneDoWhile
…
JMP DoWhile
DoneDoWhile:
MOV AX, . . . (etc.)
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Memory Declarations
 reserve memory for variables
 2 sizes:
• DB
reserves a byte of memory
• DW
reserves a word (2 bytes) of memory
 may also provide an (optional) initialization value as an
operand
Examples:
DB
; reserves one byte
X:
DB
; reserves one byte – label X is
; defined to represent the address
; of the byte
Y:
DB
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3
; reserve one byte – label Y etc.
; and initialize the byte to 3
94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Memory Declarations
Examples (continued):
DW
; reserve 2 consecutive bytes
Z:
DW
; reserves 2 bytes – label Z is defined to represent the
; address of the first byte
W:
DW 256
; reserve 2 bytes – label W etc., and initialize the bytes
; to 256 decimal (little endian !!!)
HUH: DW W
; reserve 2 bytes – label HUH etc., and initialize the
; bytes to contain the address of the variable W above
A:
B:
DB
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‘C’
; reserves 1 byte – labels A and B both represent the
: address of the byte, and initializes it to 43H
94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Instructions
 complete instruction must be on one line
 instruction mnemonic & operands
 operands
• immediate:
constant
– can use label reference, example:
W:
DW
...
MOV BX, W
• register:
• direct:
register name
[ address ]
– state address a label reference, example:
W:
DW
...
MOV AX, [ W ]
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Instructions
 Operands (continued)
• register indirect: [ register ]
– example
W:
DW
...
MOV
MOV
BX, W
AX, [ BX ]
• relative-offset (control flow)
– use label reference to identify target address
– assembler calculates actual offset
JMP
There
...
There:
etc.
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: Tool Directives
 statements that are intended to help tools
 are not assembled directly into instructions or memory
declarations
END Directive:
 directive to 2 tools: assembler and loader
 directs assembler to stop reading from .ASM file
• any subsequent statements are ignored
 operand: must be a label reference
• interpreted as specifying the address of the first
instruction to be executed
• directs loader to load specified address into IP after
loading .OBJ file
Syntax:
END
label-reference
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p86 Assembly Language: 2 Pass Assembly Process

each pass: processes all statements in .ASM file sequentially
from start to finish
1st Pass: for each statement:
1.
2.
3.
check syntax
allocate any memory needed for image
– memory declaration (DB, DW)
– instruction: opcode + operands
if includes a label definition: assign value to label and keep record of
(label, value) association in Symbol Table

if syntax errors in 1st pass, then write errors to .LST file and
stop, else ....
2nd Pass: build binary for each statement:
•
•
•
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may require calculating offsets – may result in errors – e.g. trying to
jump too far for a conditional jump (target out of range)
write results to .LST file
if no errors – write results to .OBJ file
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p86 Assembly Language: 1st Pass in Detail
Memory Allocation:
 assumes allocation starts at address 0000H
 uses location counter ($) to track address of next byte to
allocate
 as bytes are allocated, adjust $ value
 for each memory declaration: (DB, DW)
• allocate # bytes declared, for example: suppose $ = 0006H
and next line of program is: DW
– will allocate 2 bytes for DW statement:
» use address 0006H for low byte
» use address 0007H for high byte
– after allocation: $ = 0008H (next byte to allocate)
•
if next line of program is: DB ‘Hi Mom’
– will allocate 6 bytes for DB statement
» use address 0008H for ‘H’, …, 000DH for ‘m’
– after allocation: $ = 000EH
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: 1st Pass in Detail

for each instruction:
•
•

allocate bytes needed to encode opcode and operands
example: suppose $ = 0006H when next line of program is:
MOV CL, BL
– requires 2 bytes to encode instruction: opcode, CL dest, BL source
– after allocation: $ = 0008H
if next line of program is:
Instruction encoding
MOV BX, 1
details later
– requires 4 bytes to encode
– 2 bytes: opcode, BX dest, imm as src
– 2 bytes: imm src value
– after allocation: $ = 000CH
when a label definition is encountered:
•
•
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value of label = $
– value of label is the address of the next byte allocated to the
program
save (label, $-value) in Symbol Table for recall during 2nd pass
94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: 2nd Pass in Detail





build image of bytes to be loaded at addresses
memory declaration: initialization value?
• if no init value – use default?
instructions: opcodes and operands
• addressing mode info
• label references in operands: look up values to use for label
in Symbol Table
• for relative-offsets: offset = label-value – $
results to .LST file
• errors
• image created (none if errors!)
.OBJ file contains image in format that can be loaded by
simulator
• includes info about “start address” – to initialize IP during
loading
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: HI.LST Example
$
0000
0000
0004
0007
0008
000B
000C
Label definition: in 1st pass – put
binary image
value in Symbol table
start:
C7C2E904
mov
dx, 04E9H
C6C048
mov
al, 'H'
EE
out
[dx], al
C6C069
mov
al, 'i'
EE
out
[dx], al
F4
hlt
end
start
After 1st Pass:
 syntax OK
 bytes have been allocated to statements
 Symbol Table constructed:
Symbol
Value
start
0000H
After 2nd Pass:
 binary image constructed
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: ASSIGN2.LST Example
34
35
37
43
44
45
0000
0000
0004
0008
000B
000E
Start:
C7C4FEFF mov
C7C2E904 mov
C6C510
mov
C6C101
mov
8B1EA300 mov
sp,
dx,
ch,
cl,
bx,
0FFFEH
04E9H
16
1
[Value]
47 0012
48 0012 D3E3
49 0014 7206
50 0016
51 0016 C6C030
52 0019 E90300
54 001C
55 001C C6C031
Outloop:
shl
bx, cl
jc
Got1
Got0:
mov
al, ‘0’
jmp
Dump
Got1:
mov
al, ‘1’
57
58
Dump:
out
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001F
001F
EE
[dx], al
94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: ASSIGN2.LST Example
60
61
316
317
318
319
0020 80ED01
0023 75ED
Symbol
Start
Outloop
Got0
Got1
Dump
009B
009D
009F
00A1
0A00
6400
E803
1027
sub
ch, 1
jnz
Outloop
Value
Note: Do not confuse $ with
0000H
0012H
$ = artifact of assembler
0016H
IP = artifact of processor
001CH
001FH
Ten:
Hundred:
Thousand:
TenThousand:
dw
dw
dw
dw
321 00A3 AF90 Value:
dw
Symbol
Value
Ten
009BH
Hundred
009DH
Thousand
009FH
TenThousand
00A1H
Value
00A3H
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IP
10
100
1000
10000
090AFH
94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: EQU Directive



allows declaration of symbolic constants
improves (human) readability
reduces the use of “magic numbers”
Note: No “:”
Syntax:
symbolic-name EQU numeric-constant

1st Pass: records (symbolic-name, constant) in symbol table

2nd Pass: replaces every occurrence of the symbolic-name with
the specified constant
• is NOT allocated any bytes !!!
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: EQU Example
DisplayPort
EQU 04E9H ; magic number
MOV
DX , DisplayPort
; improved readability
 is assembled to same encoding as
MOV
DX , 04E9H
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: ORG Directive



allows specification of the address of the next byte to be
allocated
improves (human) readability
reduces the use of “magic numbers”
Syntax:
ORG numeric-constant

Assembler assigns location counter the value of the specified
numeric constant
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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p86 Assembly Language: ORG Example
…
JMP
ORG
DoAtF000H:
MOV
DoAtF000H
; suppose this instruction was
; assembled to bytes starting at 0120H
0F000H
BX , . . .
; MOV instruction will be assembled
; to bytes starting at address F000H


Must always increase location counter (never decrease)
Typical use:
• Force assembly to specific address range
• Resulting code will reside in particular type of memory
located at that address (e.g. ROM)
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94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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