Von Neumann Architectures and the PDP-8

A computer is an electronic ( A computer is an electronic (?) device operating under control of instructions stored in its own memory unit (stored program concept) that accepts data (input) processes data arithmetically and logically displays information (output) from the processing and/or stores the results for future use.

The von Neumann Model • The von Neumann model consists of five major components: (1) input unit; (2) output unit; (3) arithmetic logic unit; (4) memory unit; (5) control unit. Source: Computer Architecture and Organization by M. Murdocca & V. Heuring Fig 1-13 page 10

The System Bus Model • A refinement of the von Neumann model, the system bus model has a CPU (ALU and control), memory, and an input/output unit. • Communication among components is handled by a shared pathway called the system bus, which is made up of the data bus, the address bus, and the control bus. There is also a power bus, and some architectures may also have a separate I/O bus. Source: Computer Architecture and Organization by M. Murdocca & V. Heuring Fig 1-14 page 10

Seven Properties of Von-Neumann Architectures G. Blaauw and F. Brooks on page 589 of their book Computer Architecture - Concepts and Evolution list seven "salient features" of von-Neumann architectures as proposed in the 1946 paper "Preliminary discussion of the logical design of an electronic computing device" by A. Burks, H. Goldstine, and J. von Neumann. Single stream of instructions sequenced by instruction counter Instructions stored with data in addressable memory Instructions encoded as numbers - modifiable by arithmetic operations Radix 2 (binary) Word length long enough for scientific computation Single Address - single operation instructions Single Accumulator with MQ Register  

Stored Program Concept: Instructions stored with data in addressable memory Instructions encoded as numbers - modifiable by arithmetic operations Addressable Memory: Memory accessed by its numeric address (location in memory) Distinction between address of memory and contents at that address

The instruction set defines the architecture. Studying Computer Architecture: We look at three things Memory: How is memory structured/organized? What is the size in bits of the smallest addressable cell in memory? Is memory word addressable or byte addressable. What is the memory address space (size of memory)? How are physical addresses obtained from logical addresses? Processor/CPU: Registers: size and number, general purpose vs. special purpose. What ALU operations are supported? Instruction Set: What is the instructions mix? What is the instruction format and how many are there? What addressing modes are supported? The instruction set defines the architecture.

The PDP-8 Introduced in 1965 by Digital Equipment Corp. (DEC) – priced at $18K (cheap!) Transistor technology (2nd generation technology) Considered/marketed as a “mini-computer” 12–bit word-addressable; 4096 words of memory; 8 op-codes 16 bit byte-addressable very successful PDP-11 was the (market) successor

The PDP-8 Emulator Program

PDP-8 Bits Words and Integer Representation 12 bit words – bit numbering left to right msb-> <-lsb +---+---+---+---+---+---+---+---+---+---+---+---+ 0 1 2 3 4 5 6 7 8 9 10 11 normally represented in octal binary Octal octal 000 100 4 001 1 101 5 010 2 110 6 011 3 111 7

Organization of PDP-8 Memory 4096 (212) words 12 bit physical addresses memory organized into 32 pages of 128 words page/offset logical addressing scheme bits 0 – 4: page; bits 5 – 11: offset zero page /current page addressing

PDP-8 Registers 12 bit accumulator 1 bit link register for carry out +---+ +---+---+---+---+---+---+---+---+---+---+---+---+ | | | | | | | | | | | | | | | Link Accumulator 12 bit PC (program counter) holds address of next instruction 3 bit IR (instruction register) holds current op-code 12 MQ register – needed for multiplication/division 12 bit Central Processor Memory Register (CPMA) holds address to access memory 12 bit Memory Buffer Register (MB) – hold contents of memory access

Input/Output It’s complicated Problem of data format conversion How do you synchronize a fast CPU with a slow I/O device? Restricted to reading and writing single ASCII characters Uses busy waiting loop Keyboard and printer (TTY device) have data buffers and 1-bit ready flag for synchronization

Where individual bits control micro-functions PDP-8 Instructions Memory Reference Instructions 0 1 2 3 4 5 6 7 8 9 10 11 +---+---+---+---+---+---+---+---+---+---+---+---+ | opcode |IA |MP | offset address | Opcode 6 (I/O) Instructions | 1 | 1 | 0 | device number | function | Opcode 7 Microinstructions (3 sub-groups) | 1 | 1 | 1 ||0 |CLA|CLL|CMA|CML|RAR|RAL|0/1|IAC| Where individual bits control micro-functions

Addressing Modes Effective Address (Eaddr) = Address of Operand MRI instructions Effective Address (Eaddr) = Address of Operand Zero Page Addressing EAddr = 00000 + offset Current Page Addressing EAddr = Instr Address [0…4] + offset Indirect Addressing Auto-Indexing

PDP-8

Seven Properties of Von-Neumann Architectures Recall the seven Blaauw and Brooks "salient features" of von-Neumann architectures Single stream of instructions sequenced by instruction counter Instructions stored with data in addressable memory Instructions encoded as numbers - modifiable by arithmetic operations Radix 2 (binary) Word length long enough for scientific computation Single Address - single operation instructions Single Accumulator with MQ Register  

Machine language coding the PDP-8

12-bit Two’s Complement Binary Representation +---+---+---+---+---+---+---+---+---+---+---+---+ | s | b | b | b | b | b | b | b | b | b | b | b | where s has weight -211 000 000 000 000 = 0 000 000 000 001 = 1 000 000 000 010 = 2 000 000 000 011 = 3 111 111 111 111 = -1 111 111 111 110 = -2 111 111 111 101 = -3 111 111 111 100 = -4

Nine PDP-8 Machine Codes Twos Complement Add (Load Accumulator) 2 Increment and Skip (Next Instruction) if Zero Deposit and Clear Accumulator Jump Negate Accumulator 7300 Clear Accumulator and Link Halt Skip Next Instruction if Accumulator is Negative 7510 Skip Next Instruction if Accumulator is Positive