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Appendix D
Glossary of Terms

absolute address: the numerical number identifying an address.

accumulator: the main register within the microprocessor and the only one equipped for arithmetic

ACR: abbreviation for Auxiliary Control Register. One of the VIA registers.

active high: any input which requires a logic I to turn it on.

active low: any input which requires a logic 0 to turn it on.

address bus: the 16 lines from the microprocessor which activate the selected memory location or device.

address: a number which is associated with a particular memory location. This number can be in decimal or hexadecimal.

and gate: a gate which delivers a logic I out only if all inputs are logic 1.

anding: using a mask to ensure selected bits become or remain 0.

assembler mnemonics: a three-letter group uniquely defining an op code.

assembler: a program which converts a program written in assembly code to the equivalent machine code.

base address: the operand address of an indexed instruction.

base: the number of different characters used in a counting system. Decimal is base 10, binary is base 2 and hex is base 16.

bit: one of the two possible states of a binary counting system, 1 or 0.

block diagram: a simplified diagram of an electrical system using interconnected labelled boxes.

Boolean algebra: an algebraic notation, introduced by George Boole, for manipulating two-state logic.

bubble sort: sorting an array by pairs at a time until all data is in order.

bus: a collection of wires having some common purpose such as data bus, address bus and control bus.

byte: a group of 8 bits.

Centronics: trademark for a standardised parallel interface for printers.

chip: accepted slang for an integrated circuit.

compiler: system software which translates a program written in high level language into a machine code equivalent. The entire program is translated before it is run.

conditional assembly: when parts or all of the assembled code can vary depending on test conditions.

Darlington: a two-transistor configuration used to multiply the current gain.

data bus: the 8 lines from the microprocessor which carry the data to and from memory or external devices.

DDRB: abbreviation for Data Direction Register B. One of the VIA registers.

decimal: the normal counting system using the ten characters 0,1,...9.

decoder: a logic device with many possible outputs, only one of which can be activated at a time. This depends on the logic pattern applied to the 'select' inputs

direct addressing: the operand is a two-byte address as distinct from zero page addressing which is a single byte address. Also called absolute addressing.

disassembler: a program which will display a machine code program in assembly language. The opposite process to assembly.

effective address: the sum of the base and relative address.

exclusive or gate: a gate which delivers a logic I only if the inputs are at different logic states.

exclusive oring: using a mask to ensure that selected bits assume the opposite state.

firmware: programs already in ROM.

flag: a single bit used to indicate whether something has happened or not (see program status register).

handshaking: a term used to describe the method of synchronising an external device to the computer.

hardware: all the bits and pieces of a computer such as the chips, circuit board, keys, etc. That which you can see, feel and break

hex: see hexadecimal.

hexadecimal: a counting system using sixteen characters 0, 1,...9,A,B,C,D,E,F

high byte: the most significant half of a two-byte number.

high-level language: a language written in the form of statements, each statement being equivalent to many machine code instructions. BASIC is a high level language.

IER: abbreviation for Interrupt Enable Register. One of the VIA registers[FR: abbreviation for Interrupt Flag Register. One of the VIA registers.

immediate addressing: the operand is the data itself rather than an address.

implicit address: see implied address.

implied address: an address which is inherent in the op-code, therefore requiring no following operand.

index register: either the X or Y register when used to modify an address.

indexed address: an address which has been formed by the addition of an index register's contents.

indexed indirect addressing: the indirect address is the sum of the operand and contents of Y.

indirect addressing: the operand refers to an address in page zero which is the address of the wanted data.

indirect indexed addressing: the indirect address is modified by the addition of Y.

instruction register: a register within the microprocessor holding the opcode during instruction decoding.

integer: a whole number without a fraction.

integrated circuit: a chip containing a number of interconnected circuits.

interpreter: system software which translates and executes each high level language statement separately. BASIC is normally interpreted although compiler versions exist.

IRB: abbreviation for Input Register B. One of the VIA registers.

latch: a buffer register which retains old data until new data is enabled.

logic gates: electrical circuits which behave as switches. The input conditions determine whether the switch is 'open' or 'closed'.

low byte: the least significant half of a two-byte number.

low-level language: a series of codes rather than a language, each line resulting in one order to the microprocessor.

lsb: the least significant bit in the byte (the right-most bit).

LSI: large scale integration. Normally taken to mean in the order of tens of thousands of circuits on a single chip. The 6502 microprocessor is LSI.

machine code: strictly, this term should be used for instructions written in binary; now used loosely to include hex coding and assembly language.

macro: a routine assembled in line each time it is called.

mask: a bit pattern used in conjunction with either AND, EOR or ORA to act on selected bits within a byte.

merge sort: similar to bubble sort but faster due to progressive halving of the array before sorting into pairs.

microprocessor: the integrated circuit which is the central processor or 'brain' of the computer. The BBC machine uses the 6502 species.

microprogram: a program inside the microprocessor which informs it how to carry out each machine code instruction.

mnemonics: code groups chosen so we can memorise them easily.

MOB: abbreviation for Moving OBject. Any screen object which is destined to be moved,

msb: the most significant bit in the byte (the leftmost bit).

msi: medium scale integration. Normally taken to mean up to a few hundred circuits on a single chip.

nibble: a group of 4 bits.

nybble: see nibble.

object code: the translated version of the source code.

one's complement: a number formed by changing the state of all bits in a register.

op-code: abbreviation for operational code. It is that part of a machine code instruction which tells the computer what kind of action is required.

operand: that part of a machine code instruction which gives the data or where to find the data.

operating system: the software already in ROM which is designed to help you use the computer.

or gate: a gate which delivers a logic out if any one or more inputs are logic 1.

ORB: abbreviation for Output Register B. One of the VIA registers.

oring: using a mask to ensure selected bits become or remain 1.

OSBYTE: keyword for Operating System Byte. Allows machine code calls to the operating system.

OSRDCH: keyword for Operating System Read Character. A subroutine for reading a character from selected input systems.

OSWORD: keyword for Operating System W ord. Similar to OSBYTE but allows more parameters to be passed.

OSWRCH: keyword for Operating System Write Character. Passes character to selected output system.

page one address: any address within the range 256 to 511 decimal or 0100 to 01FF hex

PC: see program counter.

PCR: abbreviation for Peripheral Control Register. One of the VIA registers.

PIA: abbreviation for the 6820 Peripheral Interface Adaptor.

pixel: a small picture element.

program counter: the only 16-bit register in the 6502. Contains the address of the next instruction byte.

program status register: a register containing flag bits which indicate if overflow, carries, etc. have been caused by the previous instruction.

PSR: see program status register.

read: to examine the existing data in a register or memory location, usually by means of LDA, LDX or LDY.

relative address: the contents of the index register.

resident assembler: an assembler which is already in ROM when you purchase the machine.

resident subroutines: those in ROM which you can use, providing you know their starting address.

ROM: abbreviation for Read Only Memory. Information stored is permanent even when the power supply is off.

rotate: similar to shift but any bit pushed out from the carry is reinserted at the other end.

rpn: abbreviation for 'reverse Polish notation', which is concerned with the order in which numeric variables are processed by a machine.

RS423: a standardised interface which passes data serially along a single line.

scrolling: movement of the screen vertically or horizontally in order to bring fresh data into view.

shift: to move the bit pattern, one place to the left or right.

signed binary: the binary system which uses the msb as a sign bit.

silicon chip: most chips are fabricated from a silicon base although some of the super-fast modern varieties may be using a mixture of gallium and arsenic.

software: general term for all programs.

source code: the program in its high level form.

sprite: a screen object destined to be moved, together with accompanying coordinate data. Similar to MOB.

SR: abbreviation for Shift Register.

ssi: small scale integration. Normally taken to mean a few circuits, often simple logic gates, on a single chip.

subroutine: a program segment which will normally have general-purpose use and which can be used in other programs.

supply rail: a wire, feeding several components with a specific voltage.

symbolic address: an arbitrarily chosen name used in place of the numerical address. It is only recognised if it has been previously assigned to this number .

tristate: logic devices which can be either in the HIGH, LOW or open circuit state. When in the open circuit state, the output of the device is transparent to a common bus line.

TTL: abbreviation for Transistor Transistor Logic, a family of compatible logic chips operating on 5 volts. First launched by Texas Instruments but soon second-sourced by other manufacturers.

two-pass assembly: passing the source code twice through the assembler. Essential if branches are to forward addresses.

two's complement: a number formed by adding I to the one's complement. Used for negative number representation.

unsigned integer: a binary number without using the msb as a sign bit

user port: one of the output sockets which can be used to control your own special devices.

user subroutines: subroutines which you can make for yourself.

vector: a word in memory containing the address of an operating system routine.

VIA: abbreviation for the 6522 Versatile Interface Adaptor chip.

volatile memory: one which loses all data when power is interrupted.

write: to piace new data into a register or memory location, usually by means of STA,STX or STY. The old data is overwritten by the new.

X register: a general-purpose register which can be used in indexed addressing.

Y regisier: similar to X register.

zero page address: any address within the range 0 to 255 decimal or 00 to FF hex.


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