Topic 4Instruction Set

Microprocessor Fundamentals

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Objectives

• Define RISC and CISC– Describe the difference

• To become familiar with the AVR instruction set– Specifically the ATmega instruction set– ATmega adds a few instructions– Examine Atmel’s instruction set nomenclature– Examine both the Instruction Set Summary and the

more detailed documentation

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AVR• AVR Microcontrollers are RISC as opposed to CISC

– RISC: Reduced Instruction Set Computers• 100 – 200 instructions

» Includes all variations of addressing modes» Without variations there may be only 50 (or less) instructions

• Faster: usually executes instructions in one clock cycle• Difficult to program complex math (in assembly)• Can program in C or Java

– CISC: Complex Instruction Set Computers• Hundreds of instructions• Many special purpose• Usually includes complicated (for assembly) math functions• Can program in many high level languages

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The Status Register

• The Status Register

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Flags

• Notation used by ATMEL to inform the programmer that the instruction affects/does not affect the status register bit

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The Status Register

• C bit:– Set (set to a logic 1) if the there is a carry out or a borrow in as

a result of the operation, otherwise 0• Z bit:

– Set to a logic 1 if the result of an operation is 0, otherwise 0• N bit:

– Set to a logic 1 if the result of an operation is negative (the MSb of the result is 1), otherwise 0

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The Status Register

• V bit:– Set to a logic 1 if there is a 2’s compliment overflow

as a result of the operation, otherwise 0• Example: if two positive numbers (MSb = 0) are added

together and the result is a negative number (MSb = 1)– 7F + 1 = 80 (7F = 01111111, 1 = 00000001, 80 = 1000000)

• Example: if two negative numbers (MSb = 1) are added together and the result is a positive number (MSb = 0)

– 80 + 80 = 00 with C = 1 (80 = 1000000, 00 = 00000000)

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The Status Register

• S bit:– Tests N V after the result

• So if both N and V = 1, S =0• If only N = 1 or if only V = 1, S =1

• H bit:– Set to a logic 1 if there is a carry from b3 to b4 during

an ADD operation, otherwise 0– Other special situations set or clear H

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The Status Register

• T bit:– Transfer bit used by BLD and BST instructions

• I bit:– Global interrupt enable/disable flag

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Registers and Operands

• Notations used by ATMEL documentation

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Example Instructions

• Let’s look at a few Add instructions

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Add Instructions

• ADD• The mnemonic is ADD• The operands are the destination register (Rd) and the source

register (Rr)• The description indicates that this is an add without using

the carry bit

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Add Instructions

• ADD• The operation indicates that the values in the two registers

are added and the result is stored in the destination register (Rd)

• The Z, C, N, V, S, and H bits are affected by this instruction• The instruction takes one clock cycle to execute

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Add Instructions

• ADC• The mnemonic is ADC• The operands are the destination register (Rd) and the source

register (Rr)• The description indicates that this is an add with the carry bit

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Add Instructions

• ADC• The operation indicates that the values in the two registers

and the value of the C bit are added and the result is stored in the destination register (Rd)

• The Z, C, N, V, S, and H bits are affected by this instruction• The instruction takes one clock cycle to execute

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Add Instructions

• ADIW• This instruction is NOT as straightforward as the previous

two instructions• The mnemonic is ADIW• The operands are the destination register (Rd) and

immediate data (K) – see nomenclature on pg 11• The description indicates that this is an add immediate to

word– But, what exactly does that mean?

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Add Instructions

• ADIW (You may be asking: exactly where did I get this information?)

• The operation indicates that the register specified (Rd) is combined with the register immediately following it (Rd+1), then ….

• The 16 bit immediate data (K) is added to these two registers, then ….• The results are stored in the two combined registers

» (Rd and Rd+1)• The Z, C, N, V, and S, (but not H) bits are affected by this instruction• Takes 2 clock cycles to execute

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Add Instructions

• ADIW – This information is difficult (impossible??) to get out

of this table– There is more detailed information later in the

documentation ……

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ADIW• The documentation

provides more detailed information

• For example …..

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ADIW

• This description let’s us know that K (the immediate data) can be a number between 0 and 63

• Only the uppermost 4 register pairs can be used• Must specify the “even numbered” register• Also, this instruction is not available on all AVR chips

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ADIW

• This description provides the Boolean expressions for determining the CCR bits

• Not necessary for us to know – it just has to work right• The best information in this section are the examples

adiw r25:24,1 ; adds 1 to r25:r24adiw zm:zl,63 ; adds 63 to the Z-Pointer (r31:r30)

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MOV

• MOV is fairly straightforward• Note: It copies the value in one register to another• The value in the source register (Rr) remains unchanged

– If it got changed by this instruction it would have something like: Rr 00

• LDI is also fairly straightforward• Loads immediate data into Rd (destination register)• But: is K (the immediate data) 8 or 16 bits, or some other range of

numbers?

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LDI• LDI:

– K is 8 bits– Only registers 16

to 31 can be used• That was NOT

obvious in the summary

– No bits in the CCR are affected by this instruction

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LD

• The two LD instructions may need a closer look– LD Rd,X+– What is meant by Rd (X)?– What is meant by X X+1– LD Rd,-X– What is meant by Rd (X)?– What is meant by X X-1– What is meant by the order of these statements?

In-Class Exercise: answer these 5 questions

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LD

• The two LD instructions may need a closer look– LD Rd,X+– What is meant by Rd (X)?– What is meant by X X+1– LD Rd,-X– What is meant by Rd (X)?– What is meant by X X-1– What is meant by the order of these statements?

The data at the memory location pointed to by X is transferred to Rd

The value in X is incremented by 1 and then stored back in X

The data at the memory location pointed to by X is transferred to Rd

The value in X is decremented by 1 and then stored back in X

The order of these statements tells us which is done first. For the LD Rd,X+, first the data is transferred to Rd then X is incremented. In the LD Rd,-X, first X is decremented, then data is transferred to Rd

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Branch Instructions

• The branch instructions (in this case all are conditional) all have an operand of “k”

• What is k?

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Branch Instructions

• The branch instructions (in this case all are conditional) all have an operand of “k”

• What is k?• Can k be a label?

‘k’ is an address (as shown on page 11)

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Branch Instructions

• The branch instructions (in this case all are conditional) all have an operand of “k”

• What is k?• Can k be a label?

• The operation column shows the condition for which the branch is taken

• Example: BREQ branches if the Z bit = 1• Are BRCC and BRSH the same?

‘k’ is an address (as shown on page 11)Yes, ‘k’ can be a label – makes reading your program easier

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Branch Instructions

• The branch instructions (in this case all are conditional) all have an operand of “k”

• What is k?• Can k be a label?

• The operation column shows the condition for which the branch is taken

• Example: BREQ branches if the Z bit = 1• Are BRCC and BRSH the same?

‘k’ is an address (as shown on page 11)Yes, ‘k’ can be a label – makes reading your program easier

Yes, they both branch under the same condition (C = 0)

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BRSH and BRCC

The descriptions are a little different …..

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BRSH and BRCC

The descriptions are a little different. The operations are a little different, but essentially the same (both have “if C = 0”).

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BRSH and BRCC

The descriptions are a little different. The operations are a little different, but essentially the same (both have “if C = 0”).What really gives it away (that they are the same) is that the machine code (the 16-bit opcode) for both is the same – they both assemble to the same instruction

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BRSH and BRCC

And, another thing that becomes clear is that branch instructions can only branch 64 addresses back or 64 addresses forward.

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What’s the point?

• You cannot memorize the whole instruction set• But, the most used instructions will become so

familiar that essentially they have been memorized

• When writing a program, use the summary sheet and example programs to choose instructions

• Then, look them up, every time you use one, in the more detailed section of the documentation, until you know what happens

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Summary

• In this topic we:– Defined RISC and CISC

• And described the difference between them– Became familiar with the AVR instruction set

• Specifically the ATmega instruction set• Examined Atmel’s instruction set nomenclature• Examined both the Instruction Set Summary and the more

detailed documentation