1. CoE3DJ4 Digital Systems Design
Chapter 7: Assembly Language Programming

2. Assembly language
Assembly language is a computer language lying between extremes of machine language and high level languages (e.g., C or Pascal)
An assembly language program is a program written using labels, mnemonics and so on in which each statement corresponds to a machine instruction
Assembly language programs, often called source code or symbolic code, cannot be executed by a computer
A machine language program (object code), binary bytes representing instructions, is executable by a computer
An assembler translates an assembly language program into a machine language program

3. Assembly language Many 8051 assemblers are available
Intel’s original MCS-51 family assembler (ASM51) is no longer available but has set the standard for other assemblers
We focus on the assembly language programming using the common features of ASM51 which are supported by most assemblers.

4. Assembly language programs
Assembly language programs contain the following:
Machine instructions
Mnemonics of executable instructions
Assembler directives
Instructions to assembler defining program structure, symbols, data constants and …
Assembler controls
Set assembler modes and direct assembly flow
Comments
Enhance the readability of programs
General format of each line:
[label] mnemonic [operand][,operand][…][;comment]

5. Assembly language programs
Label field: represents the address of the instruction that follows
Used in branching to this instruction
Mnemonic field: Instruction mnemonics or assembler directives
Example: ORG, ADD, MOV
Operand field: Contains the address or data used by the instruction
A label may be used to represent the address or a symbol may be used to represent a data constant
Comment field: Remarks to clarify the program

6. Assembly language programs
Special Assembler Symbols: are used for register-specific addressing modes
Example: A, R0, DPTR, PC, C and AR0 to AR7
AR0 to AR7 represent addresses of R0 through R7 in the current register bank.
$ can be sued to refer to the address of current instruction
Example: JNB TI, $ is the same as
HERE: JNB TI, HERE

7. Generic Jumps and Calls
Assembler allows programmers to use a generic JMP instead of SJMP, AJMP, or LJMP and CALL can be used instead of ACALL or LCALL.
Assembler converts JMP to SJMP if no forward references are used and the jump destination is within -128 locations
JMP and CALL are converted to AJMP and ACALL if no forward references are used and the instruction following the JMP or CALL instruction is in the same 2K block as the destination instruction
If short or absolute forms cannot be used, the conversion is to the long form

8. Assemble-time expression evaluation
Values and constants in the operand field may be expressed with an expression (e.g., 2+3)
Use of expressions makes assembly language programs more readable and more flexible
When an expression is used, assembler calculates a value and inserts it into instruction
Expression calculations are performed using 16-bit arithmetic, however, either 8 or 16 bits are inserted into the instruction as needed

9. Assemble-time expression evaluation
Number bases: base for numerical constants is indicated by B for binary, O or Q for octal, D or nothing for decimal and H for hexadecimal.
MOV A, #15
MOV A,#1111B
MOV A, #17Q
Character String: strings of one or two characters may be used as operands. ASCII codes are converted to binary by assembler. Character constants are enclosed in single quotes (‘)
Example: MOV DPRT, #’AB’
MOV DPRT, #4142H

10. Assemble-time expression evaluation
Arithmetic operators: addition, subtraction, multiplication, division and modulo
Example: MOV A, #25 MOD 7 is the same as MOV A, #4
Logical operators: OR, AND, XOR and NOT
Example: MOV A, #’9’ AND 0FH is the same as MOV A, #9
Special operators: shift right (SHR), shift left (SHL), high-byte (HIGH) , low byte (LOW)
Example: MOV A, #HIGH 1234H and MOV A, #12H are the same

11. Assemble-time expression evaluation
Relational operators: When a relational operator is used between two operands, result is always false (0000H) or true (FFFFH). The operators are:
EQ = equals
NE <> not equals
LT < less than
LE <= less than or equal
GT > greater than
GE >= greater than or equal
Example: MOV A, #5 NE 4 is the same as MOV A, FFH

12. Assemble-time expression evaluation
Operator precedence: from highest to lowest is ()
HIGH, LOW
*, /, MOD, SHL, SHR
+, -
EQ, NE< LT, LE, GT, GE,
NOT
AND
OR, XOR
Example: ‘A’ OR ‘A’ SHL 8 will result in 4141H

13. Assembler directives
Assembler directives are instructions to assembler
They are placed in mnemonic field of program
They have no effect on the content of memory (except DB and DW)
Assembler directives:
Assembler state control
Symbol definition
Storage initialization/reservation
Segment selection
Program linkage

14. Assembler state control
ORG: set origin
Format: ORG expression
Example: ORG 100H
END: last statement in source file
No label is permitted

15. Assembler state control
Some 8051 instructions (i.e., PUSH and POP) allow only register addresses to be used.
AR0 to AR7 are used to represent addresses of R0 through R7 in the current register bank
USING: informs the assembler of the currently active register bank
By combining USING with symbols AR0 to AR7 we can specify the address of any register in register banks.
Format is: USING expression
Example:
USING 3
PUSH AR7
USING 1
First push will be assembled to PUSH 1FH whereas the second one will be assembled to PUSH 0FH
Note: USING does not actually switch register banks, it only informs the assembler of the active bank

16. Symbol definition EQU: equate
EQU assigns a numeric value to a specified symbol name
Example:
N27 EQU 27
CR EQU 0DH

17. Storage initialization/reservation
A segment is a block of code or data memory created by assembler
There are two types of segments: generic and absolute
Location counter: assembler maintains a location counter for each segment
Location counter is a pointer to the address space of active segment and represents an offset for generic segment or the actual address for absolute segment
Memory initialization and reservation directives (i.e., DS, DB, DW or DBIT) change the value of location counter as they allocated memory
ORG sets a new value for location counter

18. Storage initialization/reservation
Generic segments are created using the SEGMENT directive
Format: symbol SEGMENT segment_type
“symbol” is name of the segment
“segment_type” could be:
CODE: code segment
XDATA: external data space
DATA: internal data space accessible by direct addressing (00H-7FH)
BIT: bit space, overlapping byte locations 20H-2FH
Example: EPROM SEGMET CODE
In order to select a segment we use RSEG directive
Example: RSEG EPROM
When RSEG is used to select a segment, that segment becomes active and will be used by assembler until segment is changed with RSEG or with an absolute segment directive

19. Storage initialization/reservation
Absolute segments: reside in a fixed memory location
They are created using BSEG, CSEG, DSEG and XSEG directives
Format:
BSEG [AT address]
CSEG [AT address]
DSEG [AT address]
XSEG [AT address]
Example: BSEG AT 30H

20. Storage initialization/reservation
If an address is provided, assembler terminates the last absolute segment of the specified type (if any) and creates a new segment starting at that address
If an address is not specified, the last address of the specified type is continued
If no absolute segment of this type was previously selected and the address is omitted, a new segment is created starting at location 0
By default, assembler selects CODE segment as the active segment and initializes counter to 000H.

21. Storage initialization/reservation
Storage initialization and reservation directives initialize and reserve space in word, byte or bit units.
Space is reserved starts at the location indicated by the current value of location counter in the currently active segment
DS (define storage)
Format: [label:] DS expression
DS reserves space in byte units
It can be used in any segment type except BIT
Label represents the address of the first location of memory

22. Storage initialization/reservation
Example: Create a 40 byte data buffer starting at address 30H and fill it with zero.
DSEG AT 30H
LENGTH: EQU 40
BUFFER: DS LENGTH
MOV R7,#LENGTH
MOV R0,#BUFFER
LOOP:
INC R0
DJNZ R7,LOOP

23. Storage initialization/reservation
DBIT (define bit)
Format: [label:] DBIT expression
DBIT reserves space in bit units
It can be used only in BIT segment
Label (if used) represents the address of the first location of memory
Example:
BSEG
KBFLAG: DBIT 1

24. Storage initialization/reservation
DB (define byte)
Format: [label:] DB expression
DB initializes code memory with byte values (places data constants in code memory)
A CODE segment must be active for DB
DB permits character strings
Label (if used) represents the address of the first location of memory

25. Storage initialization/reservation
Example:
CSEG AT 0100H
SQUARES: DB 0,1,4,9,16,25
MESSAGE: DB ‘Login’
Resulting memory:
address value C F
010A 6E

26. Storage initialization/reservation
DW (define word)
Format: [label:] DW expression
DW is the same as DB except two memory locations are assigned to each data
Example:
CSEG AT 200H
DW $,‘A’,1234H,2
address value