1.5 Computer programming 15
ROM is used for storing the basic input/output operating system (BIOS). Since
information stored in a strict ROM cannot be changed, BIOS recorded cannot
be upgraded.
The programmable read-only memory (PROM) allows us to record
information after fabrication. In an erasable programmable ROM (EPROM),
data is erased and recorded using a special ultraviolet (UV) light burner. In an
electronically erasable programmable ROM (EEPROM), data is erased by con-
ventional electronic techniques. Modern hardware uses upgradable flash BIOS
recorded in the EEPROM.
Problems
1.4.1.A number is stored in four bytes. How many addresses does this number
occupy?
1.4.2.What is the maximum useful memory size of an eight-bit memory add-
ressing system in Mbytes?
1.5 Computer programming
The central processor is designed to respond to only a specific set of instructions
written in machine language and encoded as binary strings. An instruction
is composed of operation codes and accompanying arguments or parameters.
For example, the binary equivalent of instruction 67099098095 may request
addition, designated by the digits 98, of the content of the memory positioned
at the address 670 and that of the memory positioned at the address 990, placing
the sum in the memory positioned at the address 095.
Symbolic languages
Symbolic languages employ words instead of operation codes, and refer
to operations by symbolic terms such asadd. An instruction in the lowestlevel
symbolic language, called the assembly language, is translated into
the machine language code (object code) using a translation program called
theassembler. The instructions of an assembly language make reference to the
loading of variables to memory locations and fetching variables from memory
locations. The mapping of assembly language commands to machine language
instructions is one-to-one: each command is implemented by one instruction.
The assembly language implements the lowest level of communication that is
meaningful to humans.
A typical assembly command is:mov b2, 3Ah, meaning “move the hex-
adecimal value 3A to the processor register b1”; the hexadecimal representation