EECE631 Microcomputer System Design

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EECE631 Microcomputer System Design

• Spring 2008
• Lecture 2 ARM Assembly
• Chris Lewis
• clewis_at_ksu.edu

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Terminology

• Big-Endian Bit Ordering left most bit is bit 0,
  right most bit is N-1 on N bit Machine
• Big-Endian Byte Ordering - byte 0 is stored in
  the lowest address, byte N-1 is stored in the
  highest address on N byte transfer
• Little-Endian Bit Ordering
• Little-Endian Byte Ordering

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Software Terminology

• Assembly Language A programming language that
  allows the use of mnemonics in place of binary
  machine language instructions
• Assembler A program that allows the computer to
  translate assembly language to machine language
• Object file A machine language program that is
  the result of assembly process performed by an
  assembler

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Reference Material
ARM ASSEMBLY LANGUAGE PROGAMMING By Peter Knaggs
and Stephen Welsh
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Assembly Language Statement
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CPU Registers

• R1-R15 32 bit registers
• R0-R12 General Purpose Registers
• R13 Stack pointer
• R14 GPR and Link Register
• R15 Program counter and Status Register
• CPSR Current Program Status Register
• SPSR Saved Program Status Register

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CPSR, SPSR
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Program Counter

• R15 is split into two parts.
• Bits 2 to 25 are the program counter (PC).
• address of the next instruction to be fetched
• only 24 bits out of 26, 2 lowest bits always 0.
• PC0 on reset
• Normally PC with each non-branching instruction
• Status Register R15 Bits 0,2,26,27,28,29,30,31

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Assembly Program Module

• Begins w/Module Control Directive NAME
• Ends w/Module Control Directive END
• NAME main
• .
• .
• .
• END main

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ARM supports two Code types

• ARM, CODE32
• 32 bit instuction set
• CODE16
• 16, or possibly 32 bit Thumb instruction set
• Assembly directive within a module determines
  what kind of code is generated
• ARM Use 32 bit ARM machine code
• CODE32 Use 32 bit ARM machine code
• DATA Define a data segment within the code
• THUMB Use Thumb or Thumb2 machine code
• CODE16 Use Thumb or Thumb2 machine code

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Example
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Within code, there may be data
DATA assembly directive to insert data within
code
Question What happens if the PC gets set to an
address containing data? Question Can a program
modify itself?
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Segments Program Location Counter

• DATA or CODE
• Relocatable or NOT
• RSEG Relocatable segment
• ASEGN Named relocatable segment
• ASEG Absolute segment
• ORG Sets PLC
• Aligned
• ALIGNRAM increments PLC to align
• ALIGNROM fills with zeros to align
• EVEN PLC to even address
• ODD PLC to odd address

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Value Assignments

• permanent value to local module
• ALIAS permanent value to local module
• ASSIGN temporary value
• DEFINE file wide value
• EQU permanent value to local module
• LIMIT checks value against limits
• SET temporary value
• SETA temporary value
• VAR temporary value

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Example
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Conditional Directives

• IF,ELSE,ELSEIF,ENDIF

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MACRO Processing

• MACRO defines a macro
• ENDM ends the macro
• REPT repeats instructions n times
• REPTC repeats but subs chars
• REPTI repeats but subs strings
• ENDR ends repeat section

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Example
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Predefined Macro Symbols

• _agrs of arguments passed to macro

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Macro Example

• ORG 0
• B main
• base EQU 0x100
• count EQU 0x110
• init EQU 0x120
• CODE32
• PUBLIC main
• main MOV R0, 0
• REPTI a, base, count, init variable a
  gets three values, base, count, init
• LDR R1, a load R1
  with a
• STRB R0, R1, 0 store
  first byte of R0 at address of R1 offset by 0
• ENDR
• B . Branch to here, infinite loop

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Main Limitation of RISC

• Loading a 32 bit number into a 32 bit register
  with an assembly command with only 32 bits.
• Use Data Segments and offset from the PC
• The power pc can also use multiple commands
• Load upper half word
• Shift left to get into upper half
• Load lower half word
• Add or OR the two together
• LDR R0,(some defined data label)
• Assembler figures it out for you

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ARM ASSEMBLY
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ARM ASSEMBLY
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Assembly
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Assembly
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Assembly
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Assembly
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Addressing Modes
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Addressing Modes
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Addressing Modes
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Operand 2
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Condition Codes