in various sizes: from 512 to 8K words. The size will
depend on the computer and the functions of its ROM.
The arrangement of ROM uses the same concept as
main memory. A row (x)/column (y) arrangement is
used to obtain the ROM addresses. The array of the
ROM cell matrix also varies with the size of the ROM.
Certain portions of main memory are set aside for the
ROM addresses. When ROM is selected at the front
panel or as part of a start up routine for basic
input/output system (BIOS), the computer will default
to the addresses established by the manufacturer. ROM
comes in one of two basic packages: either a module or
a ROM chip. To make changes to ROM programs, you
must remove the applicable ROM module or ROM
chip(s) from the CPU module or the pcb on which they
are mounted and replace it/them with the module or
chip(s) containing the new version of the program.
ROM (NDRO) Modules
Nondestructive readout (NDRO) memories may
consist of a single pcb or several pcbs that come in a
single module (fig. 6-34). They are usually located in
a larger CPU module or as part (one pcb) of a group of
pcbs located in a chassis.
Figure 6-34.Mainframe CPU with an NDRO memory identified.