Another Real Machine: The DEC PDP-8
As the Digital Equipment Corporation PDP-8 computer, having originated with a very simple instruction set, was also inexpensive enough that it was the first computer within the reach of many organizations, it holds a place in the affections of many.
Of course, a number of other computers might also have been noted as making computing more affordable, such as the IBM 650, the Bendix G-15, the Packard Bell 250, and the Royal McBee LGP 30, for example. There was more to the enduring affection in which the PDP-8 is held than its affordability, but that is a phenomenon I will not attempt to analyze at length.
The diagram below shows an overview of the instruction formats available with the PDP-8 and related computers throughout their history:
The first column shows the basic instructions included with the PDP-5 and all PDP-8 models. The opcodes for memory-reference instructions were:
000 AND And
001 TAD Two’s Complement Add
010 ISZ Increment and Skip if Zero
011 DCA Deposit and Clear Accumulator
100 JMS Jump to Subroutine
101 JMP Jump
August 21, 2008
Another Real Machine: The DEC PDP-8
What is Assembly Language?
An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture. This representation is usually defined by the hardware manufacturer, and is based on abbreviations (called mnemonics) that help the programmer remember individual instructions, registers, etc. An assembly language is thus specific to certain physical or virtual computer architecture (as opposed to most high-level languages, which are usually portable).
Assembly languages were first developed in the 1950s, when they were referred to as second generation programming languages. They eliminated much of the error-prone and time-consuming first-generation programming needed with the earliest computers, freeing the programmer from tedium such as remembering numeric codes and calculating addresses. They were once widely used for all sorts of programming. However, by the 1980s (1990s on small computers), their use had largely been supplanted by high-level languages, in the search for improved programming productivity. Today, assembly language is used primarily for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems.
A utility program called an assembler is used to translate assembly language statements into the target computer’s machine code. The assembler performs a more or less isomorphic translation (a one-to-one mapping) from mnemonic statements into machine instructions and data. (This is in contrast with high-level languages, in which a single statement generally results in many machine instructions. A compiler, analogous to an assembler, is used to translate high-level language statements into machine code; or an interpreter executes statements directly.)
Many sophisticated assemblers offer additional mechanisms to facilitate program development, control the assembly process, and aid debugging. In particular, most modern assemblers (although many have been available for more than 40 years already) include a macro facility (described below), and are called macro assemblers.
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Key concepts
Typically a modern assembler creates object code by translating assembly instruction mnemonics into opcodes, and by resolving symbolic names for memory locations and other entities. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include macro facilities for performing textual substitution—e.g., to generate common short sequences of instructions to run inline, instead of in a subroutine.
