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28 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

Mag tape
units
CPU
Card
punch
Data
channel
Line
printer
Card
reader

Drum
Multi Data
plexor channel
Disk
Data
Disk
channel
Hyper
tapes
Memory
Data Teleprocessing
channel equipment
Figure 2.5 An IBM 7094 Configuration

from memory for an instruction fetch. Except for the occurrence of a branching in-
struction, which is typically infrequent, this means that the control unit has to access
memory for an instruction on only half the instruction cycles. This prefetching sig-
nificantly reduces the average instruction cycle time.
The remainder of the columns of Table 2.3 will become clear as the text proceeds.
Figure 2.5 shows a large (many peripherals) configuration for an IBM 7094,
which is representative of second-generation computers [BELL71]. Several differ-
ences from the IAS computer are worth noting. The most important of these is the
use of data channels. A data channel is an independent I/O module with its own
processor and its own instruction set. In a computer system with such devices, the
CPU does not execute detailed I/O instructions. Such instructions are stored in a
main memory to be executed by a special-purpose processor in the data channel it-
self.The CPU initiates an I/O transfer by sending a control signal to the data channel,
instructing it to execute a sequence of instructions in memory.The data channel per-
forms its task independently of the CPU and signals the CPU when the operation is
complete. This arrangement relieves the CPU of a considerable processing burden.
Another new feature is the multiplexor, which is the central termination point for
data channels,the CPU,and memory. The multiplexor schedules access to the memory
from the CPU and data channels, allowing these devices to act independently.
The Third Generation: Integrated Circuits

A single, self-contained transistor is called a discrete component. Throughout the
1950s and early 1960s, electronic equipment was composed largely of discrete