Transcript AVR Assembler Directives, simulator/debugger

Writing an Assembly-language
program
Atmel assembly language
CS-280
Dr. Mark L. Hornick
1
The AVR assembler
…is a modern, “single pass” assembler
 Converts mnemonics to machine instructions (opcodes +
operands)
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Ex: add r20, r5 is assembled to 0x0d45
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Case is irrelevant; alternately: ADD R20, R5
Reports syntactical errors
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Ex: ADE R20, R5 (ADE: no such instruction)
Can create listing (.lst) and map files
Generates object (.hex) files that can be executed on a
simulator or on real hardware
 Intel defined the .hex file format
Used automatically by gcc (C/C++ compiler)
 You’ll use the C compiler in CE2810
CS-280
Dr. Mark L. Hornick
2
Comments in Assembler
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In addition to the “;” style of commenting the
AVR Assembler permits C-style comments:
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Block comments starting with “/*” and ending with
“*/”
“//” comments where the remainder of the line is
ignored
These C-style comments are unusual for
Assemblers
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Typical Assemblers only permit “;”
CS-280
Dr. Mark L. Hornick
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Directives are used within an assembly
language program to control certain
aspects of the operation of the Assembler
Directives begin with a period (.)
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Directive often take operands, but not always
Ex: .ORG 0x2a
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Case does not matter
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.org 0x2A is equivalent
Directives are not instructions – they are not translated to
opcodes/operands
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They only direct the Assembler how to interpret subsequent assembly
language instructions or generate output
All directives are documented in the online help of AVRStudio
Directives usually are placed at the beginning of a program, but may
appear anywhere
CS-280
Dr. Mark L. Hornick
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Number representation in AVR
assembly language
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All values are decimal unless specified
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.ORG
.ORG
.ORG
.ORG
42
;
0x2A
;
052
;
0b00101010
decimal 42
hexadecimal
octal
; binary
Radix prefixes
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0b, 0B – binary (0b101010 or 0B101010)
0 – octal
0x, 0X – hexadecimal (0x002a or 0x2A)
CS-280
Dr. Mark L. Hornick
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Some assembler directives are used to
define the location of the program
instructions in Flash memory:
.CSEG
 Alerts the Assembler that subsequent
assembly statements are intended to
generate instructions for the Code Segment

ie, where executable machine code is placed
.ORG <n>
 Directs the Assembler where to begin
placing subsequent instructions in memory
 Example:
.CSEG ;the default segment
.ORG 0x2a
directs the subsequent machine instructions
to be placed after the first 42 (0x2a) reserved
CS-280
words
Dr. Mark L. Hornick
Reset and interrupt vector section
42 words (84 bytes)
$002A
Your program goes here!
$3C00
NNN bytes configurable
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The processor begins executing
instructions at flash memory address 0
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But the first 42 words should be reserved for special instructions
known as Reset and Interrupt Vectors
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Unless you explicitly put an instruction at address 0, the
(invalid) opcode 0xFFFF is placed there
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Reset and interrupt vector section
42 words (84 bytes)
$002A
The processor “skips” the invalid opcode and moves onto the next
address location
To avoid this, use the .CSEG directive to place an instruction at
address 0 that forces the processor to jump to a location where
valid instructions exist
Your program goes here
.CSEG;the default segment
.ORG 0x0
RJMP 0x2A ; jump to app section
.ORG 0x2A
; your program’s instructions
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The first reserved word at address 0 is the Reset Vector that is
designed to contain the instruction that gets executed whenever
the CPU is reset
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i.e., a jump to where the actual program begins
The operand of RJMP is the address to jump to
CS-280
Dr. Mark L. Hornick
$3C00
1024 words (2048 bytes)
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Some Assembler directives can be
used to define variable-like symbols
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.DEF <symbol>=R<n>
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Define a symbol to refer to a specific register
.DEF Counter=R10
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Counter can be used anywhere in the program in place of R10
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Case does not matter
Placement of .DEF does not matter, but should precede first usage
Symbols can be redefined
Use .UNDEF <symbol> to undefine a symbol
.EQU <constant>=<expression>
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Define a constant to be used in place of a constant value
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.EQU START= 0x42
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Ex: .ORG START
.EQU ZERO = 0
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Ex: ADD R20, Counter
Ex: LDI R20, ZERO
constants cannot be redefined or undefined
.SET <variable>=<expression>
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Same as .EQU, but variables defined with .SET can be changed later
CS-280
Dr. Mark L. Hornick
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File-related
Assembler Directives
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.LIST (.NOLIST)
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Enable (disable) list file generation during
assembly
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List files have the .lst file extension)
On by default
.INCLUDE <“file”>
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Include the contents of another file
Ex: .INCLUDE “m32def.inc”
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Includes a file that contains numerous convenient
.EQU and .DEF directives
CS-280
Dr. Mark L. Hornick
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AVR Debugger/Simulator
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Simulates execution of the compiled program
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Start, stop, single-step
Run to breakpoint
Can view contents of memory, registers
Tracks time required to execute
Can see IO port status
CS-280
Dr. Mark L. Hornick
10
Labels can be used in place of
actual addresses
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Every input line can be preceded by a label
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an alphanumeric string terminated by a colon (:)
Labels are used as targets for jump and
branch instructions
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The assembler automatically figures out what
address to assign to a label
CS-280
Dr. Mark L. Hornick
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Every input line can be
preceded by a label
Example:
.CSEG
.ORG 0x0
rjmp Start_of_program ;go to beginning
.ORG 0x2A
Start_of_program:
add r1, r2
Here:
rjmp Here ;repeat forever
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Labels are alphanumeric strings terminated by a colon(:)
Labels are given the value of the location counter at the place they
appear
Labels can be used as jump targets in program instructions
The label Start_of_program: is assigned the value of the address of
beginning of the program (0x2A); Here: is assigned 0x2B
The assembler automatically figures out what address to assign to a
label
CS-280
Dr. Mark L. Hornick
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