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Instructions From the Bottom Up

Based in part on material from Chapters 4 & 5 in Computer

Architecture by Nicholas Carter

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Code from the bottom up

• When one examines a problem starting with the smallest constituent pieces and starts putting them together, then one is said to be taking a “bottom up” approach.

• Let us consider programming on a computer that has the simple architecture shown on the next slide from the bottom up.

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Input port 1 Accumulator

ALU FlagsInput port 2

Prog. counter

Mem.Add.Reg.

Memory

MDR

Instr. Reg.

Control

C

B

TMP

Output port 3

Output port 4

Display

Keyboard encoder

Bus

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Lots of registers

• After Memory, Control and the ALU, most of the other items in this architecture are registers.– Recall counter is a register that can increment

• Registers are small units of memory that are associated with the processor.

• The registers serve various special purposes.• In some cases, main memory could be used in

place of a particular register, but using the register speeds up the processor.

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Speeding up the process

• Registers are faster than main memory because writing to or reading from a block of memory is (at least) a two-step process – Specify the address– Read or write a value at that address

• A register is a single unit of memory and thus eliminates the first step.

• (There is also a vicinity consideration, the registers are on the processor chip, the memory is on a separate chip or chips.)

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Incidental remark on memory speed

• There are two basic types of random access memory (RAM)– Static RAM (SRAM) – does not require refreshing to

hold its value and is fast but expensive– Dynamic RAM (DRAM) – does require refreshing to

hold its value and is slow but cheap

• Special blocks of SRAM (cache) can be used to speed up the processor’s interaction with memory. In addition, some cache is right on the microprocessor chip.

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Register size

• The size of the registers (the number of bits, binary digits, it has) is an important feature of a processor.

• A register may hold – Data: thus its size may affect the range and/or precision

of numbers available.

– Address: thus its size may affect the number of addressable locations.

– Instruction: thus its size may affect the number of instructions one can have.

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Intel family chip comparison table from howstuffworks.com

It’s been at 32 bits for awhile so one doesn’t hear a lot about it, but 64 is already here for high-end processors.

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Register hiding

• Registers play a role in programming at the assembly level, but their use is hidden when one programs with high-level languages. – (C allows for some low-level programming.)

• This is an example of those familiar ideas – Layering– Information hiding

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Input port 1 Accumulator

ALU FlagsInput port 2

Prog. counter

Mem.Add.Reg.

Memory

MDR

Instr. Reg.

Control

C

B

TMP

Output port 3

Output port 4

Display

Keyboard encoder

Bus (Architecture reminder)

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Controlling a register• One enters a value into a register (i.e. loads

it) – If the load control input is active– When the clock is at the appropriate part of its

cycle (e.g. positive edge)

• If a register is allowed to place its value on the bus, it will have an enable control input. It will do so – When the enable control input is active

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Other control inputs

• The other components in the architecture also have control pins that determine whether the component is active and what specific action it might be performing.

• These control inputs are connected to the control unit which determines at each instance what components are active.

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Micro-coding Load Accumulator A

• To discuss coding from the bottom up, one starts with micro-code.

• Micro-code is “burned” into the read-only memory (ROM) of the control unit and is sent out to the control inputs of the other components.

• Let us examine the micro-code of the assemble level instruction Load Acc. A – Assume control inputs are active when they are high

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Input port 1 Accumulator

ALU FlagsInput port 2

Prog. counter

Mem.Add.Reg.

Memory

MDR

Instr. Reg.

Control

C

B

TMP

Output port 3

Output port 4

Display

Keyboard encoder

Bus (Architecture reminder)

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Address State: the value of the program counter (which recall is the address of line of the program to be performed) is put into memory address register.

Control pins

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Increment State: the program counter is incremented, getting it ready for the next time.

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Memory State: the current line of the program is put into instruction register (so Control knows what to do).

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Recall the instruction consisted of a load command and an address. A copy of the address is now taken over to the memory address register.

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The value at that address is loaded into Accumulator A.

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For the load command, there is no activity during the sixth step. It is known as a "no operation" step (a "no op" or "nop").

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• These 1’s and 0’s are MICRO-CODE.

• It is fed directly to the hardware (specifically the control pins of the devices).

• Unless you are a hardware manufacturer, you usually don’t program at this level known as microprogramming.

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ROM, PROM, EPROM, EEPROM

• ROM: Read Only Memory (written by manufacturer)

• PROM: Programmable Read Only Memory (written by user, write once)

• EPROM: Erasable Programmable Read Only Memory (user can erase, take out of computer and apply ultraviolet light)

• EEPROM: Electrically Erasable Programmable Read Only Memory (user can erase with out removing from computer)

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Moving up one level

• Micro-code rarely changes thus it is written in (“burned into”) read-only memory (ROM).

• By the true meaning of the terms, ROM is RAM, by which I mean that a particular “word” in ROM is accessed using an address and does not require progressing through previous locations.

• Thus we can access the micro-code by supplying its address. Effectively, the addresses of the micro-code is the next level of programming: machine code.

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Machine code• In a machine-code program, one tells the computer to

execute the micro-code found at Control-ROM Address1, then execute the micro-code found at Control-ROM Address2, etc.

• The program looks something like – Address1– Address2

except that the addresses are in binary (1’s and 0’s). So it might look like– 00101001– 00011010

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Machine code is software

• Micro-code is often referred to as firmware to indicate that it is the transition between hardware and software, that it is written in ROM and not changed very readily.

• Machine code is placed in RAM, which is easily rewritten. Thus it is soft.

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Ground floor for software

• Machine code is the lowest level of software.

• It is the only level of software capable of communicating with the hardware (via the firmware).

• Thus all software must be put into machine language form before it can be executed.

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What makes software soft?

• Part of the flexibility of software is that the set of instructions can be rearranged, leading to a different program.

• Another part of the flexibility is the inclusion of data with the instructions. – A machine level instruction may be to move the value stored

at a certain memory location to a register (e.g. Accumulator A).

– Part of the instruction is data, the memory location, which can be changed to yield different results.

– Also the data at the memory location can be changed to yield different results.

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Operator/Operand

• So a software instruction can specify an action and data to be acted upon. – We use the term operator (also op-code) to refer to the

action.

– We use the term operand to refer to the data.

• So if we instruct the computer to copy the value from memory location 35 to accumulator A, then – Copying a value from memory to Acc. A is the operator

– Location 35 is the operand

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Instruction set

• So a programmer can modify the order of instructions and the data portion (operands) of those instructions, but what is fixed is the set of allowable instructions, known as the instruction set.

• The instruction set is an important characteristic of the processor, what instructions and how many are allowed.

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Instruction Set Size Issue

• Often one can realize a complicated instruction by using simple instructions, but then one must execute several instructions instead of just one. – The burden is taken off the hardware and put on the

software. • So an issue in processor design is whether one should have

a smaller collection of simpler, faster instructions or whether one should support a larger collection of more complex instructions.

• If the more complex instructions are rarely used, it may be better to leave them out provided their effect can obtained using a combination of other instructions.

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RISC/CISC• Reduced Instruction Set Computers (RISC) use

processors that limit the number of instructions with the idea that the supported instructions will be simpler and faster. – A simpler instruction set also means less circuitry,

fewer transistors and thus cheaper processors.

• To distinguish conventional designed from RISC machines, the term Complex Instruction Set Computers (CISC) was introduced.

• Ideas from both camps have merged, so now it is hard to distinguish RISC from CISC.

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RISCs in Parallel

• RISC processors are coming around again but this time in parallel.

• The simpler RISC processor requires fewer transistors and takes up less space. Thus one can place two or more of them on a single chip.

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Stored program as data

• At the machine language level, one is writing software by entering strings of 1’s and 0’s. This is tedious and error prone.

• In the stored program concept, the instructions and the data are placed together in memory.

• It is then possible to treat the program as data – either of a different program or indeed of the same program. – The latter case is known as self-modifying code.

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Assembly: one small step …

• Instead of coding in machine language’s binary strings, people introduced mnemonics for the instructions. – For example, LDA 35 instead of something like

00101001 00100011.

• Then there was a program that treated the mnemonic code as data and created from it the corresponding machine code.

• The program is called an assembler and the mnemonic code is called assembly.

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One-to-one

• There is essentially a one-to-one correspondence between assembly code and machine code. The assembler code corresponds to a look-up table. – It looks up the assembly code and outputs the

corresponding machine code. – It can also do some checking. If the code

submitted is not proper assembly code, …

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Hardware Aware

• The assembly programmer has to have knowledge of the computer’s architecture– What registers there are– What actions are allowed on those registers– Etc.

• If the hardware changes, the assembly program may no longer be any good.

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Backward Compatibility• So as to not render customer’s software completely

useless, processor designers take backward compatibility into consideration.

• A computer is called “backward compatible” if it can run the same software as the previous model of the computer. – The reverse is not necessarily true for software written

based on the new hardware.• A manufacturer will try to make a series or family of

processors backward compatible provided it doesn’t stand in the way of progress.

• Sometimes the computer has a special mode for running old programs.

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Intel• Intel introduced the first microprocessor (the CPU on a

single chip) – the 4004 designed in conjunction with the Japanese firm Busicom for use in a calculator.

• They followed this with the 4040, 8008, 8080, 8086, 8088, and the 80286.

• At this stage, people drop the 80 at the beginning, giving the 286, 386, 486.

• Because a company cannot trademark a number, they stopped using this naming scheme, so instead of 586, we got the Pentium.

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Intel

• Intel, founded in 1968 by Bob Noyce and Gordon Moore is the world's largest manufacturer of computer chips.

• Recently it has been challenged by AMD (Advanced Micro Devices) and Cyrix (now part of VIA).

• Nearly all PCs are based on Intel's x86 architecture, in fact that’s how some people define the term PC.

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Speaking of Gordon Moore

• Moore observed an exponential growth (effectively doubling every couple years) in the number of transistors per integrated circuit and predicted that this trend would continue.

• This is known as Moore’s Law. • There are similar predictions about

memory, price, etc.

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 Year of

introductionTransistors

4004 1971 2,250

8008 1972 2,500

8080 1974 5,000

8086 1978 29,000

286 1982 120,000

386™ processor 1985 275,000

486™ DX processor 1989 1,180,000

Pentium® processor

1993 3,100,000

Pentium II processor

1997 7,500,000

Pentium III processor

1999 24,000,000

Pentium 4 processor

2000 42,000,000

     

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Portability• Instead of insisting on hardware’s backward

compatibility, one can try to make software portable.

• In describing software, portable means that the software can be run on a variety of computers. Another term used is independence, in this case machine or hardware independence. Assembly’s use of specifics of the registers makes it machine dependent.

• One has to program at a higher level to gain machine independence.

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High-level language

• Languages which are above assembly language, known as high-level languages, hide the details of the hardware and thus are portable or hardware independent.

• Of course, high-level language are not directly executed. There is an in-between process or processes (compiling, translating, interpreting, linking, etc.) necessary before they can be executed.

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Generations• Another way of discussing this is to talk about

generations. • 1GL or first-generation language is machine

language.• 2GL or second-generation language is assembly. • 3GL or third-generation language is a "high-level"

programming language, Fortran, C, Java, Visual Basic, etc.

• 4GL or fourth-generation language is a more descriptive language, closer to human language usually used to query databases.

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Instruction Set

• One aspect of processor design is to determine what instructions will be supported.

• There must be rules (syntax) for how instructions are expressed so that the code can be parsed, one instruction distinguished from the next, the data (operand) separated from the action (operator).

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Number of operands

• An example of syntax would be whether an operator is binary or unary

• Binary operators take two operands– e.g. the boolean operator AND– (x<y) AND (i=j)

• Unary operator take one operand– E.g. the boolean operator NOT– NOT(x<y)

• Some ops are context dependent– - 5 (unary) versus 4-5 (binary)

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Operation versus Instruction

• An operation such as addition, which is binary (takes two operands), can be coded using instructions that are unary (one operand) if there is a default location implied such as Accumulator A.

• 5 + 6 becomes – Load 5 (places data in acc. A)– Add 6 (adds number in Acc. A to new number)

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Where does the answer go?

• Whether it’s 5 + 6 or (Load 5, Add 6), there’s the question of what to do with the result.

• We can again use the default location of Accumulator A to place the answer in or we can include a third operand that indicates where the result should be placed.

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Store• Placing the result of an operation in memory is known

as storing it. Thus with unary instructions, we would have – Load 5– Add 6– Store 7

• The operand of the store is an address indicating where to store the answer, which is held in Accumulator A.

• Or one might have just one instruction with three operands– Add 7, 5, 6

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Addressing Modes

• The “7” in the previous example was clearly an address. But this raises the question as to what were the operands in the two previous instructions (Load and Add).

• For those instructions, the operand might have been the actual numbers one wanted added or the addresses of numbers one wanted added.

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More than one kind of add

• By Add one typically means the latter case on the previous slide. The operand is not the number to be added but the address of the number to be added.

• A designer can include a distinct add instruction, Add Immediate, in which the operand is the actual number to be added.

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Add Indirect

• In another version of addition, the operand is an address, and the data at that address is also an address, and the actual number to be added is located at the second address.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: XXX

The arrow indicates the program counter, we assume it has not executed the statement it points to.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 5

The Load 4 instruction has been executed. The value at location 4 (which is a 5) has been loaded into the accumulator.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 12

The Add Indirect 5 instruction has been executed. One goes to location 5 to find a value of 6. That 6 is an address, thus one goes to location 6 to find a value of 7 and that is added to the 5 waiting in the accumulator. The result of 12 is placed in the accumulator.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 18

The Add Immediate 6 instruction has been executed. The value 6 is data which is added to the 12 waiting in the accumulator. The result of 18 is placed in the accumulator.

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The Stop Instruction• Recall that in what is actually executed (machine

code) the instructions themselves are numbers. • Thus it is crucial to know within a single

instruction which numbers correspond to an operation and which numbers are operands.

• Similarly on the level of the program itself, the processor needs to know where the program ends as there may be data stored after it.

• In a machine with an operating system, it is more a notion of returning (control) than of stopping or halting.

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Recap so far• So there were issues about the number of

operands. – Recall that we have a fetch-execute cycle – first an

instruction is retrieved from memory and then acted upon.

– With unary instructions adding two numbers and storing the result required three instructions, that’s three fetches and three executions.

– With ternary instructions it can be done with one instruction, one fetch and one execute. The execution is now more complicated but we have saved time on fetches.

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Recap so far (Cont.)

• More operators means more complicated circuitry, the load and store aspects of the instruction would have to built into each separate instruction.

• There is a speed versus complexity issue. And complexity also brings the issue of cost along with it.

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Recap so far (Cont.)• After determining the number of operands, came

the issue of what the operands mean. – Are they data, addresses of data, or addresses of

addresses of data?

• Either we can decide to support all of these and choose complexity. Or we can choose to support only some of them and sacrifice efficiency. – You can eliminate Add Immediate if you always store

the values you want to add.

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Data Types

• Apart from addressing, another issue is the type of data the operation is acting on. – The process for adding integers is different from the

process for adding floating point numbers.

– So one may have separate ADD and FADD for the addition of integers and floats respectively.

– Furthermore, one may need to add instructions to convert from one type to another.

• To add an integer to a float, convert the integer to a float and then add the floats.

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References

• Computer Architecture, Nicholas Carter

• Digital Computer Electronics, Albert P. Malvino and Jerald A. Brown

• http://www.webopedia.com

• http://www.intel.com