DT131B
Embedded Systems Programming
Lecture 6: Instruction Set and Assembly Programming
Dawit Mengistu (dawit.mengistu@hkr.se)
Why Assembly?
• Assemblerinstructions(code)translatelinebylineto executable machine code
– No extra loops – No extra code
• Becauseonlynecessarycodestepsareexecuted, assembly programs are sometimes the only way to meet real-time requirements.
• Thedurationofexecutionforeverylineofcodeis known.
– Time critical applications, are often written in assembler for better precision in time measurements.
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Why Assembly?
• Codeisoptimizedandcompactenoughtofit into memory constrained microcontrollers (compared with C or other HLL).
• Assemblyinstructionsaresometimestheonly way to access system resources (registers, memory, I/O.)
• Assembly languages are easier to learn. (Learn only the instruction set of the microcontroller.)
• Easiertotestanddebug.
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Why NOT Assembly?
• One can write code only using primitive CPU instructions (arithmetic, logic, shift).
• Data size depends on processor (8-bit?, 16-bit?, etc.)
• Time consuming if used to write code for mathematical tasks involving trigonometric, logarithmic, exponential, etc., functions.
• Instruction set is architecture or processor dependent. Not possible to write code that is portable across different architectures.
• Difficult to maintain.
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Hierarchy of Languages
The Instruction Set
• The complete collection of instructions that are understood by a certain CPU or processor architecture.
• Is the machine code in human readable or hexadecimal (binary) format
• Usually represented by assembly codes because machine code is not ‘human readable’
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Elements of an Instruction
• Operation code (Op code) – Do this,
• Source Operand reference – To this.
• Result Operand reference – Put the answer here
• Next Instruction Reference
– When you have done that, do this…
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Review: Instruction Cycle
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Instruction (Opcode) Representation
• In machine code, each instruction has a unique bit pattern
• For human consumption (programmers) a symbolic representation is used
– e.g. ADD, SUB, LOAD
• Opcodes and operands can also be
represented in this way
– ADD A,B
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Instruction Types
• Data processing
• Data storage (main memory) • Data movement (I/O)
• Program flow control
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Types of Operands
• Addresses • Numbers
– Integer/floating point • Characters
– ASCII etc.
• Logical Data
– Bits or flags
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Number of Operands
• Number of addresses used in an instruction.
• Example:
• A 3 address machine uses 3 addresses – Operand 1, Operand 2, Result
– a = b + c;
– Not common
– Needs very long words to hold everything
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Number of Operands (cont’)
• 2 addresses : (used in a 2 address machine) – One address doubles as operand and result –a = a + b
– Reduces length of instruction
– Requires some extra work
• Temporary storage to hold some results
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Number of Operands (cont’)
• 1 address: (used in called 1 address machines) – Implicit second address
– Usually a register (accumulator)
– Common on early machines
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Number of Operands (cont’)
• 0 (zero) addresses
– All addresses implicit – Uses a stack
– e.g. push a
push b add pop c
–c = a+ b
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Design Decision: How Many Addresses?
• More addresses means
– More complex (powerful?) instructions
– More registers
• Inter-register operations are quicker
– Fewer instructions per program
• Fewer addresses
– Less complex (powerful?) instructions – More instructions per program
– Faster fetch/execution of instructions
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Design Decisions (cont’)
• Registers
– Number of CPU registers available
– Which operations can be performed on which registers?
• Addressing modes (later…) • RISC v CISC
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RISC vs CISC
• RISC:ReducedInstructionSetComputers – 100 –200 instructions
– Includes all variations of addressing modes
– Faster: usually executes instructions in one clock cycle – Difficult to program complex math
• CISC:ComplexInstructionSetComputers – Hundreds of instructions
– Many special purpose, complicated math functions
– Suitable for programming in many high level languages – Mostly appropriate for general purpose computers
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Types of Operations
• Data Transfer • Arithmetic
• Logical
• Conversion
• I/O
• System Control
• Transfer of Control
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Data Transfer Instructions
• Specifyaddress:couldbearegisterormemory – Source address
– Destination address – Amount of data
• Maybedifferentinstructionsfordifferent movements
• Oroneinstructionanddifferentaddresses
• Highlydependentonprocessortype(x86,ARM, AVR, etc.)
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Addressing Modes
• Register addressing – transfers a byte or word from the source register (R2) to the destination register (R1)
MOV R1, R2 ;R1 = R2
• Immediate addressing: – transfers an immediate byte
or word of data into the destination register
MOV R1, 3456h ;R1 = 0x3456
• Direct addressing: moves a byte or word between a
memory location and a register
MOV R1, [1234h] ;1234h is a reachable memory address
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Addressing Modes (cont’)
• Register Indirect: (base relative or indexed) – transfers a byte or word of data between a register and the memory location addressed by an index or base register
MOV R1, [RB] ;Get data from address indexed by RB
• Base Plus Index: (base relative indexed) – transfers a byte or word of data between a register and the memory location addressed by a base register plus index register
MOV R1, [RB + RI] ;Get data from address indexed by RB+RI
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Addressing Modes (cont’)
• Register Relative – transfers a byte or word of data between a register and the memory location addressed by an index or base register plus displacement
MOV R1, [RB + 1000h] ;RB is a base register
Combinations of the above addressing modes are also possible.
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Instruction Types
• Arithmetic
– Add, Subtract, Multiply, Divide
– Signed Integer
– May include
• Increment (a++)
• Decrement (a–) • Negate (-a)
• Logical
– Bitwise AND, OR, XOR, NOT, – Comparison CMP or CP
• ShiftandRotate(nextslide)
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Shift and Rotate Operations
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Instruction Types (cont’)
• DataMovement
– Between registers (like assignment)
– From registers to regular memory or stack memory – From memory to registers
• Input/output
– May be specific instructions (e.g. IN, OUT)
– Often uses specific registers
– May be done using data movement instructions (memory mapped)
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Transfer of Control Operations
• Branch mem_addr (or Jump)
– Unconditionallytransferscontroltoinstructionatmem_addr.
• Conditional Branching:
– BZmem_addr(branchifzero)
• branch to mem_addr if result is zero
– BRNZmem_addr(branchifNOTzero)
• branch to mem_addr if result is not zero
– Other forms of branching may also exist (BRE,BRNE, BRL, BRNLE…)
• Subroutine call (CALL mem_addr) – Controltransferstomem_addr
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Branch Instruction
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Stack Usage
• Datamovementinstructionsandsubroutine calls affect the content and position of the stack.
• Aspecialregistercalledstackpointertracksthis storage
• Becareful!StackoperationsuseLIFOfordata storage and retrieval
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Byte Order
• What order do we read numbers that occupy more than one byte
• e.g. (numbers in hex to make it easy to read)
• 0X12345678 can be stored in 4x8bit locations as follows
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Byte Order (example)
Address
184
185
186
186
Value (1) Value(2)
12 78 34 56 56 34 78 12
i.e. read top down or bottom up?
• This problem is called endian-ness.
• The system on the left has the least significant byte in the lowest address: called big-endian
• The system on the right has the least significant byte in the highest address: called little-endian
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Assembly Programming
• Use hexadecimal rather than binary
– Codeasseriesoflines
– Needtoreadinstructionsetandmachinecodetranslations
• Add symbolic names or mnemonics for instructions instead of hexadecimal coding.
• Typical instruction has the following format: mnemonic operand1, operand2
• Optionally, add comment for readability and documentation
Ex: add R0, R1 ;add x and y
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Programming the AVR
• We shall use the ATMega168 and 328 series microcontrollers.
• These are 8-bit AVR processors.
• Have 32 registers (R0, R1, …, R31)
• The last six registers (R26 – R31) serve as index registers as well.
• Have one status register, stack pointer
• Nodivisionarithmetic!
• Refertothedatasheettolearntheassembler.
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Programming the AVR (cont’)
• We use the Atmel Studio IDE for writing and testing AVR assembly codes.
• TheIDEhasapowerfuldebuggerandsimulator. • Withthesimulator,youcan:
– test your assembler code without running your code on a real microcontroller;
– See the execution time of your code;
– See the contents of your program and data memory
– See the contents of important registers such as: • Status register
• Program counter • Stack pointer, etc.
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AVR Instruction Set
The following notation is used in the datasheet.
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AVR Instruction Examples
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Instruction Examples (cont’)
Data Movement Examples
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Instruction Examples (cont’)
Branch Instructions Examples
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The Status Register (SREG)
Each bit location stores important information about the just completed instruction/operation.
The contents of these fields are used for control (Branching)
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Programming Examples -1
• Write an AVR assembly code to:
– Perform the task in the C code below:
int X, Y, Z, A, B, C; X = 45;
Y = 32;
Z = X – Y;
A = Z * 2; B = Y / 2; C = A * B;
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Programming Examples – 2
• Write an AVR assembly code to:
1. Calculate the sum of N 8-bit integers stored in N
consecutive memory locations.
2. Write subroutine (function) that performs the above task.
3. Write a subroutine to calculate the factorial of a number
4. Write a subroutine to arrange numbers in ascending order.
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References
• Inthezipfile(AVR.zipunderResources)
– Atmel AVR 8-bit Instruction Set Manual
– Beginners Introduction to the Assembly Language of Atmel AVR Microprocessors Gerhard Schmidt, 2017 http://www.avrasmtutorial.net
• Barnett,CoxandO’Cull(2007),EmbeddedC Programming and the Atmel AVR. (Chapter 2)
• AdditionalReading
– William Stallings, Computer Organization and Architecture 8th ed. (2010) – Chapter 12