COMP3221: Microprocessors and Embedded Systems

COMP3221: Microprocessors and Embedded Systems

Lecture 11: Assembly

http://www.cse.unsw.edu.au/~cs3221

Lecturer: Hui Wu

Session 2, 2005

COMP3221/9221: Microprocessors and Embedded Systems

2

Overview

• Pseudo Instructions

• Macro

• Assembly Process


3

Assembly Language Format

An input line takes one of the following forms :

[label:] directive [operands] [Comment] [label:] instruction [operands] [Comment] Comment Empty line

A comment has the following form:

; [Text]

Items placed in braces are optional. The text between the comment-delimiter (;) and the end of line (EOL) is ignored by the Assembler.


4

Memory Segments

• Different types of memory are known as segments to the assembler

• Assembler directives enable code/data to be placed into different segments

• AVR has Data segment (SRAM)

Can’t place data here, just reserve space (for variables)

Code segment (Flash) Can place code or constant data here

EEPROM Segment Can place constants here

5

• From AVRStudio Help

• These arefor the AVRStudioAssembler

Directive Description

BYTE Reserve byte to a variable

CSEG Code Segment

CSEGSIZE Program memory size

DB Define constant byte(s)

DEF Define a symbolic name on a register

DEVICE Define which device to assemble for

DSEG Data Segment

DW Define Constant word(s)

ENDM, ENDMACRO End macro

EQU Set a symbol equal to an expression

ESEG EEPROM Segment

EXIT Exit from file

INCLUDE Read source from another file

LIST Turn listfile generation on

LISTMAC Turn Macro expansion in list file on

MACRO Begin macro

NOLIST Turn listfile generation off

ORG Set program origin

SET Set a symbol to an expression

Pseudo Instructions

6

Pseudo Instructions

• .byte: Reserve space; only allowed in dseg

• Segment directives .cseg and .dseg allow the text and data segments to be built up in pieces:

.dseg

amount: .byte 2

.cseg

formula: inc r0

.dseg

count: .byte 2

.db: Initialize constant in code or EEPROM segment

.dw: As above but defines a 16-bit word

7

Pseudo Instructions

• .def: Make a definition for registers only.def ZH=r31.def ZL=r30

• .device: Specify the exact processor that this program is designed for.device AT90S8515

Prohibits use of non-implemented instructions

• .macro, .endm: Begin and end macro definition• .include: Include a file• .exit: Stop processing this file

8

Expressions

• Expressions can consist of operands, operators and functions. All expressions are internally 32 bits.

• Example: ldi r26, low(label + 0xff0)

Function Operands Operator


9

Operands

• User defined labels which are given the value of the location counter at the place they appear. • User defined variables defined by the SET directive • User defined constants defined by the EQU directive • Integer constants: constants can be given in several formats, including

Decimal (default): 10, 255 Hexadecimal (two notations): 0x0a, $0a, 0xff, $ff Binary: 0b00001010, 0b11111111 Octal (leading zero): 010, 077

• PC - the current value of the Program memory location counter.

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OperatorsSymbol Description! Logical Not~ Bitwise Not - Unary Minus* Multiplication/ Division+ Addition- Subtraction<< Shift left>> Shift right< Less than<= Less than or equal> Greater than>= Greater than or equal== Equal!= Not equal& Bitwise And^ Bitwise Xor| Bitwise Or&& Logical And|| Logical Or

Same meanings

as in c


11

Functions

• LOW(expression): Returns the low byte of an expression • HIGH(expression): Returns the second byte of an expression • BYTE2(expression): The same function as HIGH • BYTE3(expression): Returns the third byte of an expression • BYTE4(expression): Returns the fourth byte of an expression • LWRD(expression): Returns bits 0-15 of an expression • HWRD(expression): Returns bits 16-31 of an expression • PAGE(expression): Returns bits 16-21 of an expression • EXP2(expression): Returns 2 to the power of expression • LOG2(expression): Returns the integer part of log2(expression)


12

Functions (Cont.)

• Examples

cp r0, low(-13167) cpc r1, high(-13167) brlt case1

… case1: inc r10

…


13

Macros

• Assembler programmers often need to repeat sequences of instructions several times• Could just type them out – tedious• Could just copy and paste - then the specializations are often forgotten or wrong• Could use a subroutine, but then there is the overhead of the call and return instructions• Macros solve this problem• Consider code to swap two bytes in memory: lds r2, p lds r3, q sts q, r2 sts p, r3


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Macros • Swapping p and q twice • Without macro

lds r2, plds r3, qsts q, r2sts p, r3


• With macro.macro myswap


.endmacro

myswapmyswap


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• There are up to 10 parameters

Indicated by @0 to @9 in the macro body

@0 is the first parameter, @1 the second, and so on

• Other assemblers let you give meaningful names to parameters

AVR Macro Parameters


16

AVR Parameterised Macro

• Without macro lds r2, p

lds r3, qsts q, r2sts p, r3

lds r2, rlds r3, ssts s, r2sts r, r3

• With macro

.macro change

lds r2, @0

lds r3, @1

sts @1, r2

sts @0, r3

.endmacro

change p, q

change r, s


17

Another Example

• Subtract 16-bit immediate value from 16 bit number stored in two registers

.MACRO SUBI16 ; Start macro definition subi @1,low(@0) ; Subtract low byte sbci @2,high(@0) ; Subtract high byte .ENDMACRO ; End macro definition

.CSEG ; Start code segment SUBI16 0x1234,r16,r17 ; Sub.0x1234 from

; r17:r16• Useful for other 16-bit operations on an 8-bit processor


18

Two Pass Assembly Process

• We need to process the file twice

• Pass One

– Lexical and syntax analysis: checking for syntax errors

– Record all the symbols (labels etc) in a symbol table

– Expand macro calls

• Pass Two

– Use the symbol table to substitute the values for the symbols and evaluate functions.

– Assemble each instruction

• i.e. generate machine code

19

An Example.include "m64def.inc"

.equ bound =5

.def counter =r17

.dseg

Cap_word:.byte 5

.cseg

rjmp start ; Interrupt vector tables starts at 0x00

.org 0x003E ; Program starts at 0x003E

Low_word: .db "hello“

start:

ldi zl, low(Low_word<<1) ; Get the low byte of the address of "h"

ldi zh, high(Low_word<<1) ; Get the high byte of the address of "h"

ldi yh, high(Cap_word)

ldi yl, low(Cap_word)

clr counter ; counter=0


20

An Example (Cont.)

main:

lpm r20, z+ ; Load a letter from flash memory

subi r20, 32 ; Convert it to the capital letter

st y+, r20 ; Store the capital letter in SRAM

inc counter

cpi counter, bound

brlt main

loop: nop

rjmp loop

21

An Example (Cont.)• Pass 1: Lexical and syntax analysis

Symbol Value

bound 5

counter 17

Cap_word 0x0000

Low_word 0x003E

start 0x0041

main 0x0046

loop 0x004c

Symbol Table

22

An Example (Cont.)

• Pass 2: code generation.

Program address Machine code Assembly code

0x00000000: C040 rjmp start …

0x0000003E: 6568 “he” ; Little endian 0x0000003F: 6C6C “ll” 0x00000040: 006F “o” 0x00000041: E7EC ldi zl, low(Low_word<<1) 0x00000042: E0F0 ldi zh, high(Low_word<<1) 0x00000043: E0D0 ldi yh, high(Cap_word) 0x00000044: E6C0 ldi yl, low(Cap_word) 0x00000045: 2711 clr counter

…


23

Absolute Assemblers

• The only source file contains all the source code of the program

• Programmers use .org to tell the assembler the starting address of a segment (data segment or code segment)

• Whenever any change is made in the source program, all code must be assembled.

• A downloader transfers an executable file (machine code) to the target system.

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Absolute Assemblers (Cont.)

Source file with location information (NAME.ASM)

Absolute assembler

Executable file (NAME.EXE)

Loader Program

Computer memory

Absolute Assembler Operation


25

Relocatable Assemblers

• The program may be split into multiple source files

• Each source file can be assembled separately

• Each file is assembled into an object file where some addresses may not be resolved

• A linker program is needed to resolve all unresolved addresses and make all object files into a single executable file


26

Relocatable Assemblers (Cont.)

Source file 1 (MODULE1.ASM


Relocatable assembler


Object file1 (MODULE1.OBJ



27

Linker

• Takes all object files and links them together and locates all addresses

• Works together with relocatable assembler

28

Linker (Cont.)







Linker program

Library of object files (FILE.LIB)

Executable file (NAME.EXE)

Code and data location

information


29

Loader

• Puts an executable file into the memory of the computer.

• May take many forms.

Part of an operating system.

A downloader program that takes an executable file created on one computer and puts it into the target system.

A system that burns a programmable read-only memory (ROM).


30

Reading

1. Chap. 5. Microcontrollers and Microcomputers

2. User’s guide to AVR assembler

– This guide is a part of the on-line documentations accompanied with AVR Studio. Click help in AVR Studio.

Documents

COMP3221: Microprocessors and Embedded Systems