Varian Data 620/i System Reference Manual page 18

W bus. The memory word (W) register is directly connected
to all memory modules through the W bus. The bus is
bidirectional and time-shared among memory modules.
L bus. The memory address (L) register is directly connected
to all memory modules through the L bus. The bus is
unidirectional.
2.1.5 Input/Output Bus
The standard DATA 620/i is provided with a bidirectional
input/output (I/O) bus that permits programmed data transfers
between periphera I devi ces and the computer.
2.1.6 Memory
The internal storage of the computer consists of 4096-word
modules connected to the Land W buses. The mainframe
can accommodate one 4096-word module. Additiona I
modules are added in an additional frame that is attached to
the mainframe. The computer memory can be expanded to
a maximum of 32,768 words using 4096-word modules.
I nstruction words read from memory are transferred to the
control section for execution. Words may be transferred,
under program control, from memory to the arithmetic/logic
section, to the operational registers, or to the I/O bus.
Words may be transferred, under program control, to memory
from the operational registers or the I/O bus. When the
direct memory access option is used, the system is capable
of direct transfer between memory and peripheral devices
on the I/O bus, concurrent with computations.
2.1.7 Direct Memory Access
The direct-memory-access (DMA) option allows data transfer
into or out of memory modules without disturbing the con-
tents of the operational registers. Only the Land W registers
are altered. Access to memory using the DMA facility is
on a IIcycle-steal" basis and requires 2.7 microseconds of
processor ti me per tra nsfer .
2. 1 .8 Mi era-EXEC
The Micro-EXEC option is a unique hardware technique for
microstep sequencing of the computer. This option provides
hardware logic in which all computer control signals are
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