Beginnings of a revolution - The Computing Universe: A Journey through a Revolution - page 6

Fig. 1.8 The first programmers of ENIAC
were women. In those days, program-
ming meant setting all switches and
rewiring the computer, a tedious opera-
tion that often took days to complete.

“First Draft of a Report on the EDVAC.” Although von Neumann had left blank
spaces on his draft for the names of co-authors, unfortunately for Ekert and
Mauchly, Goldstine went ahead and released the paper listing von Neumann as
the sole author. The report contained the first description of the logical struc-
ture of a stored-program computer and this is now widely known as the von
Neumann architecture ( Fig. 1.9 ).
The first great abstraction in the report was to distinguish between the
computer hardware and software. On the hardware side, instead of going into
detail about the specific hardware technology used to build the machine, von
Neumann described the overall structure of the computer in terms of the basic
logical functions that it was required to perform. The actual hardware that
performed these functions could be implemented in a variety of technologies -
electromechanical switches, vacuum tubes, transistors, or (nowadays) modern
silicon chips. All these different technologies could deliver the same computa-
tional capabilities, albeit with different performance. In this way, the problem
of how the logical components are put together in a specific order to solve a
particular problem has now been separated from concerns about the detailed
hardware of the machine. This splitting of responsibilities for the hardware
design and for the programming of the machine was the beginning of two
entirely new engineering disciplines: computer architecture and software
engineering.
For the hardware of the machine, von Neumann identified five functional
units: the central arithmetic unit (CA), the central control unit (CC), the mem-
ory (M), the input (I), and the output (O) ( Fig. 1.10 ). The CA unit carried out all
the arithmetic and logical operations, and the CC unit organized the sequence
of operations to be executed. The CC is the conductor , since it coordinates the
operation of all components by fetching the instructions and data from the
memory and providing clock and control signals. The CA's task is to perform
the required calculations. The memory was assumed to store both programs
and data in a way that allowed access to either program instructions or data.
The I/O units could read and write instructions or data into and out of the
computer memory directly. Finally, unlike the ENIAC, which had used decimal
arithmetic, von Neumann recommended that the EDVAC use binary arithmetic

Fig. 1.9 A Hungarian postage stamp that
honors John von Neumann, complete
with the mathematician's likeness and a
sketch of his computer architecture.