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1

Assembly

Programming (II)

Lecturer: Sri Parameswaran

Notes by: Annie Guo

2

Lecture overview

YAssembly program structure

YAssembler directives

YAssembler expressions

YMacro

YMemory access

YAssembly process

YFirst pass

YSecond pass

3

Assembly program structure

YAn assembly program basically consists of

YAssembler directives

YE.g. .def temp = r15

YExecutable instructions

YE.g. add r1, r2

YAn input line in an assembly program takes

one of the following forms :

Y[label:] directive [operands] [Comment]

Y[label:] instruction [operands] [Comment]

YComment

YEmpty line

4

Assembly program structure

(cont.)

YThe label for an instruction is associated with

the memory location address of that instruction.

YAll instructions are not case sensitive

Y Y 5

Example

; The program performs ; 2-byte addition: a+b; .def a_high = r2; .def a_low = r1; .def b_high = r4; .def b_low = r3; .def sum_high = r6; .def sum_low = r5; mov sum_low, r1 mov sum_high, r3 add sum_low, r2 adc sum_high, r3

Two comment lines

Empty line

Six assembler directives

Four executable instructions

6

Comments

YA comment has the following form:

Y;[Text]

YItems within the brackets are optional

YThe text between the comment-delimiter(;)

and the end of line (EOL) is ignored by the assembler. 7

Assembly directives

YInstructions to the assembler are created for

a number of purposes:

YFor symbol definitions

YFor readability and maintainability

YAll symbols used in a program will be replaced by the real values when assembling

YE.g. .def, .set

YFor program and data organization

YE.g. .org, .cseg, .dseg

YFor data/variable memory allocation

YE.g. .db, .dw

YFor others

8

NOTE: All directives must be preceded by a period

Summary of

AVR Assembler

directives 9

Directives for symbol

definitions Y.def

YDefine an alias for a register

YE.g. .def temp = r17 YSymbol temp can be used instead of r17 anywhere in the program after the definition .def symbol = register 10

Directives for symbol

definitions (cont.) Y.equ

YDefine symbols for values

YNon-redefinable. Once set, the symbol cannot be

redefined to other value later in the program YE.g. .equ length = 2 YSymbol length with value 2 can be used anywhere in the program after the definition .equ symbol = expression 11

Directives for symbol

definitions (cont.) Y.set

YDefine symbols for values

YRe-definable. The symbol can be changed to

represent other values later in the program. YE.g. .set input = 5 YSymbol input with value 5 can be used anywhere in the program after this definition and before its redefinition. .set symbol = expression 12

Program/data memory

organization

YAVR has three different memories

YData memory

YProgram memory

YEEPROM memory

YThe three memories correspond to three

memory segments to the assembler:

YData segment

YProgram segment (or Code segment)

YEEPROM segment

13

Program/data memory

organization directives

YMemory segment directives specify which

memory segment to use

Y.dseg

YData segment

Y.cseg

YCode segment

Y.eseg

YEEPROM segment

YThe .org directive specifies the start address

to store the related program/data. 14

Example

.dseg ; Start data segment vartab: .byte 4 ; Reserve 4 bytes in SRAM ; from address 0x200 .cseg ; Start code segment ; default start location is 0x0000 const: .dw 10, 0x10, 0b10, -1 ; Write 10, 16, 2, -1 in program ; memory, each value takes ; 2 bytes. mov r1,r0 ; Do something 15

Data/variable memory

allocation directives

YSpecify the memory locations/sizes for

YConstants

YIn program/EEPROM memory

YVariables

YIn data memory

YAll directives must start with a label so that

the related data/variable can be accessed later. 16

Directives for Constants

YStore data in program/EEPROM memory

Y.db

YStore byte constants in program/EEPROM memory

ƒexpr* is a byte constant value

Y.dw

YStore word (16-bit) constants in program/EEPROM

memory

YLittle endian rule is used

ƒexpr* is a word constant value

17

Directives for Variables

YReserve bytes in data memory

Y.byte

YReserve a number of bytes for a variable

Yexpr is the number of bytes to be reserved.

Label: .byte expr

18

Other Directives

YInclude a file

Y.include

YStop processing the assembly file

Y.exit

YBegin and end macro definition

Y.macro

Y.endmacro

YWill be discussed in detail later

19

Implement data/variables

YWith those directives, you can

implement/translate data/variables into machine level descriptions

YAn example of translation by WINAVR is

given in the next slide. 20

Sample C program

// global variables: const char g_course[] = "COMP"; char* g_inputCourse = "COMP"; char g_a; static char g_b; int main(void) { // local variables: const char course[] = "COMP9032"; char* inputCourse = "COMP9032"; char a; static char b; char i; char isCOMP9032 = 1; for(i=0; i<9; i++){ if (inputCourse[i] != course[i]) { isCOMP9032 = 0; i = 9; } } return 0; }

21

Memory mapping after build

and run 22

Memory mapping after

executionquotesdbs_dbs14.pdfusesText_20

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