Show More Show Less. No ratings or reviews yet. Be the first to write a review. Best Selling in Nonfiction See all. Open Borders Inc. Burn after Writing by Sharon Jones , Paperback 2. Save on Nonfiction Trending price is based on prices over last 90 days. You may also like. The result was the Expert line of drives, introduced in early ; the idea worked, Western Digital regained respect in the press and among users despite a recall in It was the first commercially successful minicomputer, with over 50, examples being sold over the model's lifetime, its basic design followed the pioneering LINC but had a smaller instruction set, an expanded version of the PDP-5 instruction set.
Systems returned to a faster parallel implementation but use much less costly transistor—transistor logic MSI logic.
The acme of foolishness
Intersil sold the integrated circuits commercially through to as the Intersil family. By virtue of their CMOS technology they had low power requirements and were used in some embedded military systems; the chief engineer who designed the initial version of the PDP-8 was Edson de Castro , who founded Data General. The PDP-8 combined low cost, simplicity and careful engineering for value; the greatest historical significance was that the PDP-8's low cost and high volume made a computer available to many new customers for many new uses. Its continuing significance is as a historical example of value-engineered computer design.
The low complexity brought other costs, it made programming cumbersome, as is seen in the examples in this article and from the discussion of "pages" and "fields". Much of one's code performed the required mechanics, as opposed to setting out the algorithm.
An Introduction to System Programming — Based on the PDP11
For example, subtracting a number involved computing its two's complement adding it; some ambitious programming projects failed to fit in memory or developed design defects that could not be solved. For example, as noted below, inadvertent recursion of a subroutine would produce defects that would be difficult to trace to the subroutine in question; as design advances reduced the costs of logic and memory, the programmer's time became more important.
Subsequent computer designs emphasized ease of programming using larger and more intuitive instruction sets. Most machine code was generated by compilers and report generators; the reduced instruction set computer returned full-circle to the PDP-8's emphasis on a simple instruction set and achieving multiple actions in a single instruction cycle, in order to maximize execution speed, although the newer computers have much longer instruction words.
Clark and C. However, software could do multiple-precision arithmetic. An interpreter was available for floating point operations, for example, that used a bit floating point representation with a two-word mantissa and one-word exponent. An optional memory-expansion unit could switch banks of memories using an IOT instruction; the memory was magnetic-core memory with a cycle time of 1. The PDP-8 was designed in part to handle contemporary text.
Six-bit character codes were in widespread use at the time, the PDP-8's twelve-bit words could efficiently store two such characters. In addition, a six-bit teleprinter code called the teletypesetting or TTS code was in widespread use by the news wire services, an early application for the PDP-8 was typesetting using this code. It had only three programmer-visible registers: A bit accumulator , a prog. Bus computing In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers.
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This expression covers all related hardware components and software, including communication protocols. Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical bus. Modern computer buses can use both parallel and bit serial connections, can be wired in either a multidrop or daisy chain topology , or connected by switched hubs, as in the case of USB.
An early computer might contain a hand-wired CPU of vacuum tubes, a magnetic drum for main memory, a punch tape and printer for reading and writing data respectively. In both examples, computer buses of one form or another move data between all of these devices.
In most traditional computer architectures, the CPU and main memory tend to be coupled. A microprocessor conventionally is a single chip which has a number of electrical connections on its pins that can be used to select an "address" in the main memory and another set of pins to read and write the data stored at that location.
In most cases, the CPU and memory share signalling operate in synchrony; the bus connecting the CPU and memory is one of the defining characteristics of the system, referred to as the system bus. It is possible to allow peripherals to communicate with memory in the same fashion, attaching adaptors in the form of expansion cards directly to the system bus. This is accomplished through some sort of standardized electrical connector, several of these forming the expansion bus or local bus. However, as the performance differences between the CPU and peripherals varies some solution is needed to ensure that peripherals do not slow overall system performance.
Many CPUs feature a second set of pins similar to those for communicating with memory, but able to operate at different speeds and using different protocols. Others use smart controllers to place the data directly in memory, a concept known as direct memory access. Most modern systems combine both solutions; as the number of potential peripherals grew, using an expansion card for every peripheral became untenable. This has led to the introduction of bus systems designed to support multiple peripherals.
Common examples are the SATA ports in modern computers, which allow a number of hard drives to be connected without the need for a card. However, these high-performance systems are too expensive to implement in low-end devices, like a mouse. This has led to the parallel development of a number of low-performance bus systems for these solutions, the most common example being the standardized Universal Serial Bus.
All such examples may be referred to as peripheral buses, although this terminology is not universal.
The BLISS programming language: a history
In modern systems the performance difference between the CPU and main memory has grown so great that increasing amounts of high-speed memory is built directly into the CPU, known as a cache. In such systems, CPUs communicate using high-performance buses that operate at speeds much greater than memory, communicate with memory using protocols similar to those used for peripherals in the past; these system buses are used to communicate with most other peripherals, through adaptors, which in turn talk to other peripherals and controllers.
Such systems are architecturally more similar to multicomputers, communicating over a bus rather than a network. In these cases, expansion buses are separate and no longer share any architecture with their host CPU. What would have been a system bus is now known as a front-side bus. Given these changes, the classical terms "system", "expansion" and "peripheral" no longer have the same connotations. Other common categorization systems are based on the bus's primary role, connecting devices internally or externally, PCI vs. SCSI for instance. However, many common modern bus systems can be used for both.
Internal data buses are referred to as a local bus, because they are intended to connect to local devices; this bus is rather quick and is independent of the rest of the computer operations. The external bus, or expansion bus, is made up of the electronic pathways that connect the different external devices, such as printer etc. Buses can be parallel buses, which carry data words in parallel on multiple wires, or serial buses, which carry data in bit-serial form. The addition of extra power and control connections, differential.
The Q-bus is a less expensive version of Unibus using multiplexing so that address and data signals share the same wires; this allows both a physically smaller and less-expensive implementation of the same functionality. Over time, the physical address range of the Q-bus was expanded from 16 to 18 and 22 bits. Block transfer modes were added to the Q-bus.
Byte addressing means that the physical address passed on the Unibus is interpreted as the address of a byte-sized quantity of data. Because the bus contains a data path, two bytes wide, address bit is subject to special interpretation and data on the bus has to travel in the correct byte lanes. A strict Master-Slave relationship means that at any point in time, only one device can be the Master of the Q-bus; this master device can initiate data transactions which can be responded to by a maximum of one selected slave device. At the end of the bus cycle, a bus arbitration protocol selects the next device to be given mastery of the bus.
Asynchronous signaling means; these devices use handshake signals to control the timing of the data cycle. Timeout logic within the master device limits the maximum allowed length of any given bus cycle. Eight or 16 DBAL lines are re-used for the data portion of each bus cycle. Newer generations of the bus allow block mode transfer where a single bus address can be followed by more than one data cycle. Because the address portion of each bus cycle can not transfer data, the use of block mode means fewer address cycles and more time for data cycles, allowing increased bus data transfer bandwidth.
Interrupts can be delivered to the Interrupt Fielding Processor at any of four interrupt priority levels.
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Within a given level, the cards closer to the IFP take priority over cards further back on the bus. Interrupts are vectored: a card requesting an interrupt has its interrupt vector read by the IFP. Asynchronous signaling is used but all responsibility for de-skewing of addresses and data is the responsibility of the current bus master, minimizing the complexity of the bus slave devices.
The responsibility for timing-out failed bus cycles is placed in the master devices. The complexities of handling interrupt transactions are concentrated into the single Interrupt-Fielding Processor in the system; the design of the Q-bus was closely related to the design of the Unibus both in spirit and in detailed implementation. Adapters were available from Digital and from third parties that allow Q-bus devices to be connected to Unibus-based computers and vice versa.
It is implemented by central processing microprocessors used in PDP minicomputers , it was in wide use during the s, but was overshadowed by the more powerful VAX architecture in the s.
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Sixteen-bit words are stored little-endian. Thirty-two-bit data—supported as extensions to the basic architecture, e. So a loaded PDP had 28K words. The lowest vectors are service routines.
pdp | Dave Cheney
Traps occur on some program errors, such as an attempt to execute an undefined instruction; the article PDP describes how the bit logical address space became an insurmountable limitation. During the life of the PDP, the following techniques were used to work around the limitation: Later-model PDP processors included memory management to support virtual addressing; the physical address space was extended to 18 or 22 bits, hence allowing up to KB or 4 MB of RAM.
Programming techniques, such as overlaying a block of stored instructions or data with another as needed, can conceal paging issues from the application programmer. For example, the Modula-2 compiler produces code under which the run-time system swaps 8 Kb pages into memory as individual procedures receive control; the CPU contains eight general-purpose bit registers. Register R7 is the program counter.
Although any register can be used as a stack pointer, R6 is the stack pointer used for hardware interrupts and traps. R5 is used to point to the current procedure call frame. Kernel and User modes have separate memory maps, separate stack pointers. Most instructions allocate six bits to specify an operand.
Three bits select one of eight addressing modes, three bits select one of the eight general registers; the use of three-bit groups makes octal notation natural. In the following sections, each item includes an example of how the operand would be written in assembly language for a prototypical single-operand instruction with symbol OPR.
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