RAM (cont.) 20 bits of address Address Data input Write 2 20 bytes

RAM (cont.) 20 bits of address Address Data input Write 2 20 bytes of RAM (1 Mega-byte) 8 bits (1 byte) of data Data Output

RAM (cont.) When you talk about the memory of a computer, most often you’re talking about its RAM. If a program is stored in RAM, that means that a sequence of instructions are stored in consecutively addressed bytes in the RAM. Data values (variables) are stored anywhere in RAM, not necessarily sequentially Both instructions and data are accessed from RAM using addresses RAM is one (crucial) part of the computer’s overall architecture

Computer Architecture Central Processing Unit CPU Input/ Output Devices Bus RAM

Computer Architecture CPU Keyboard Display Bus RAM Hard Disk CD-ROM

The Bus What is a bus? It is a simplified way for many devices to communicate to each other. Looks like a “highway” for information. Actually, more like a “basket” that they all share. CPU Bus Keyboard Display

The Bus CPU Bus Keyboard Display

The Bus Suppose CPU needs to check to see if the user typed anything. CPU Bus Keyboard Display

The Bus CPU puts “Keyboard, did the user type anything?” (represented in some way) on the Bus. CPU Keyboard Display Bus “Keyboard, did the user type anything?”

The Bus Each device (except CPU) is a State Machine that constantly checks to see what’s on the Bus. CPU Keyboard Display Bus “Keyboard, did the user type anything?”

The Bus Keyboard notices that its name is on the Bus, and reads info. Other devices ignore the info. CPU Keyboard Display Bus “Keyboard, did the user type anything?”

The Bus Keyboard then writes “CPU: Yes, user typed ‘a’.” to the Bus. CPU Keyboard Bus “CPU: Yes, user typed ‘a’.” Display

The Bus At some point, CPU reads the Bus, and gets the Keyboard’s response. CPU Keyboard Bus “CPU: Yes, user typed ‘a’.” Display

Computer Architecture CPU Keyboard Display Bus RAM Hard Disk CD-ROM

Inside the CPU The CPU is the brain of the computer. It is the part that actually executes the instructions. Let’s take a look inside.

Inside the CPU (cont.) Memory Registers Register 0 Register 1 Register 2 Temporary Memory. Computer “Loads” data from RAM to registers, performs operations on data in registers, and “stores” results from registers back to RAM Register 3 Remember our initial example: “read value of A from memory; read value of B from memory; add values of A and B; put result in memory in variable C.” The reads are done to registers, the addition is done in registers, and the result is written to memory from a register.

Inside the CPU (cont.) Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 For doing basic Arithmetic / Logic Operations on Values stored in the Registers

Inside the CPU (cont.) Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register To hold the current instruction

Inside the CPU (cont.) Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register Instr. Pointer (IP) To hold the address of the current instruction in RAM

Inside the CPU (cont.) Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register Instr. Pointer (IP) Control Unit (State Machine)

The Control Unit It all comes down to the Control Unit. This is just a State Machine. How does it work?

The Control Unit Control Unit State Machine has very simple structure: 1) Fetch: Ask the RAM for the instruction whose address is stored in IP. 2) Execute: There are only a small number of possible instructions. Depending on which it is, do what is necessary to execute it. 3) Repeat: Add 1 to the address stored in IP, and go back to Step 1 !

The Control Unit is a State Machine Fetch Add Exec Store Load Goto Exec Exec Exec Exec Add 1 to IP

A Simple Program Want to add values of variables a and b (assumed to be in memory), and put the result in variable c in memory, I.e. c a b Instructions in program – – – – Load a into register r1 Load b into register r3 r2 r1 r3 Store r2 in c

Running the Program r1 r2 r3 r4 IR IP 2 a 1 c 3 b 2 Logic Load a into r1 2005 CPU Memory Load a into r1 Load b into r3 r2 r1 r3 Store r2 into c 2005 2006 2007 2008

Running the Program r1 r2 r3 r4 IR IP 2 a 1 c 3 b 2 3 Logic Load b into r3 2006 CPU Memory Load a into r1 Load b into r3 r2 r1 r3 Store r2 into c 2005 2006 2007 2008

Running the Program r1 r2 r3 r4 IR IP 2 a 1 c 3 b 2 5 3 Logic r2 r1 r3 2007 CPU Memory Load a into r1 Load b into r3 r2 r1 r3 Store r2 into c 2005 2006 2007 2008

Running the Program r1 r2 r3 r4 IR IP 2 a 1 c 3 b 2 5 3 Logic Store r2 into c 2008 CPU Memory Load a into r1 Load b into r3 r2 r1 r3 Store r2 into c 2005 2006 2007 2008

Running the Program r1 r2 r3 r4 IR IP 2 a 5 c 3 b 2 5 3 Logic Store r2 into c 2008 CPU Memory Load a into r1 Load b into r3 r2 r1 r3 Store r2 into c 2005 2006 2007 2008

Putting it all together Computer has many parts, connected by a Bus: CPU Keyboard Display Bus RAM Hard Disk CD-ROM

Putting it all together The RAM is the computer’s main memory. This is where programs and data are stored. CPU Keyboard Display Bus RAM Hard Disk CD-ROM

Putting it all together The CPU goes in a never-ending cycle, reading instructions from RAM and executing them. CPU Keyboard Display Bus RAM Hard Disk CD-ROM

Putting it all together This cycle is orchestrated by the Control Unit in the CPU. Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register Instr. Pointer (IP) Control Unit (State Machine)

Back to the Control Unit It simply looks at where IP is pointing, reads the instruction there from RAM, and executes it. Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register Instr. Pointer (IP) Control Unit (State Machine)

Putting it all together To execute an instruction, the Control Unit uses the ALU plus Memory and/or the Registers. Memory Registers Register 0 Register 1 Arithmetic / Logic Unit Register 2 Register 3 Instruction Register Instr. Pointer (IP) Control Unit (State Machine)

Programming

Where we are Examined the hardware for a computer – – – – Truth tables Logic gates States and transitions in a state machine The workings of a CPU and Memory Now, want to program the hardware

Programs and Instructions Programs are made up of instructions CPU executes one instruction every clock cycle Modern CPUS do more, but we ignore that Specifying a program and its instructions: Lowest level: Machine language Intermediate level: Assembly language Typically today: High-level programming language

Specifying a Program and its Instructions High-level programs: each statement translates to many instructions Load a into r1 E.g. c a b to: Load b into r3 r2 r1 r3 Store r2 into c Assembly language: specify each machine instruction, using mnemonic form E.g. Load r1, A Machine language: specify each machine instruction, using bit patterns E.g. 1101101000001110011

Machine/Assembly Language We have a machine that can execute instructions Basic Questions: What instructions? How are these instructions represented to the computer hardware?

Complex vs Simple Instructions Computers used to have very complicated instruction sets – this was known as: CISC Complex Instruction Set Computer Almost all computers 20 years ago were CISC. 80s introduced RISC: RISC Reduced Instruction Set Computer

Complex vs Simple Instructions RISC Reduced Instruction Set Computer Fewer, Less powerful basic instructions But Simpler, Faster, Easier to design CPU’s Can make “powerful” instructions by combining several wimpy ones Shown to deliver better performance than Complex Instruction Set Computer (CISC) for several types of applications.

Complex vs Simple Instructions Nevertheless, Pentium is actually CISC ! Why?

Complex vs Simple Instructions Nevertheless, Pentium is actually CISC ! Why: Compatibility with older software Newer application types (media processing etc) perform better with specialized instructions The world has become too complex to talk about RISC versus CISC

Typical Assembly Instructions Some common assembly instructions include: 1) “Load” – Load a value from RAM into one of the registers 2) “Load Direct” – Put a fixed value in one of the registers (as specified) 3) “Store” register to the RAM Store the value in a specified 4) “Add” Add the contents of two and put the result in a third register registers

Typical Assembly Instructions Some common instructions include: 5) “Compare” If the value in a specified register is larger than the value in a second register, put a “0” in Register r0 6) “Jump” If the value in Register r0 is “0”, change Instruction Pointer to the value in a given register 7) “Branch” register is larger change If the value in a specified than that in another register, IP to a specified value

Machine Languages Different types of CPU’s understand different instructions Pentium family / Celeron / Xeon / AMD K6 / Cyrix (Intel x86 family) PowerPC (Mac) DragonBall (Palm Pilot) StrongARM/MIPS (WinCE) Many Others (specialized or general-purpose) They represent instructions differently in their assembly/machine languages (even common ones) Let’s look instructions for a simple example CPU