The PROWLER Hardware section
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//// How Many Bits is Jaguar?
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(And what does it mean anyway?)
The number of bits used to characterize a computing system is, in
general, a fairly straightforward issue. The candidates for the number
to use are:
1) The number of bits in the data bus. This is the number of bits that
can be transferred from one part of the system to another at one
time.
2) The number of bits in the ALU (Arithmetic Logic Unit).
This is the number of bits that can be computed at once.
3) The number of bits in a data register. This is the number
of bits that make up a value when stored inside a chip.
The number usually used is the first one, the width of the data bus.
This is because there are bottlenecks in all systems. These need to be
avoided. The transfer of data is a vital aspect of performance and it
has been seen to be an important way to judge overall system
performance.
First, a bit of history. The first popular microprocessor was the
Intel 8080. It had an 8-bit data bus (meaning 8 bits of data could be
transferred at one time), an 8-bit ALU (Arithmetic Logic Unit, so it
could compute 8 bits at once), and 8-bit wide registers. There was a
universal agreement that this should be called an "8-bit processor".
Another popular 8-bit processor is the 6502. Like the 8080, the 6502
has an 8-bit data bus, an 8-bit ALU and 8-bit registers. There was
also universal agreement that this should be called an 8-bit
processor. The 6502 is the processor used as the computation base of
many popular systems. These include: game systems such as the Atari
2600, 5200, 7800 and the Nintendo Entertainment System (NES), as well
as computer systems such as Atari 400, 800, Commodore VIC-20, C-64 and
Apple II. These are the classic 8-bit machines.
Now the confusion starts. Consider the Intel 8088. The chip has an
8-bit data bus, a 16-bit ALU and 16-bit registers. Intel called this
an 8-bit processor because it has an 8-bit data bus to distinguish it
from the 8086 which is the 16-bit bus version. The 8088 was used in
the first IBM PC and IBM called it 16-bit. This was probably because
the Apple II was 8-bit and IBM wanted to be bigger. In this case, the
stretch is not completely ridiculous because the 8088 is identical to
the 8086 except for the width of the data bus.
Next consider the Motorola 68000. This has a 16-bit data bus, a 16-bit
ALU and 32-bit registers. It is also known as a 16-bit processor. The
68000 was the heart of the original Apple Macintosh. For reasons no
doubt similar to IBM's, Apple chose to call the Mac a 32-bit machine
(although no one really really believed them).
Intel actually worked a clever way around this problem when the 386
came out. The 386 is a true 32-bit chip with a 32-bit data bus, a
32-bit ALU and 32-bit registers. Intel also produced a version of the
386 that was identical except that it has a 16-bit data bus. Intel
called the 32-bit bus version the 386DX and the 16-bit version the
386SX. The performance difference between these two chips demonstrates
the validity of the data bus criterion. By the convention, the 8088
and 8086 should be called the 8086SX and 8086DX respectively. The
concept of system architecture is quite useful here. The 386 is a
32-bit architecture and the 386SX is a 16-bit implementation of that
architecture. Just to add confusion, Intel changed the meaning of DX
and SX with the 486. The 486SX is missing an on-chip coprocessor.
In the video game arena, things were quite a bit simpler. The Sega
Genesis used, as it main processor, the 68000. Either, as a result of
a failure of marketing imagination or due to Apple's failure to
convince anyone that the original Mac was 32-bit, Sega called the
Genesis a 16-bit machine. The Super Nintendo Entertainment System
(SNES) is based on a 65816, a 16-bit version of the 6502. This is also
a 16-bit system. Both of these systems use 16-bit graphic chips. This
means that the graphics chips in the system are capable of moving data
around the system 16-bits at a time. The NEC Turbo Graphics 16 is an
interesting machine. It used an 8-bit processor but contained 16-bit
graphics hardware. This is the first popular example of the bit size
of a system not being determined by the central processor, but by
other parts of the system. Another example of a 16-bit system
containing an 8-bit processor is the Atari Lynx.
To recap the discussion above, there are several different numbers
that can be used to classify a computing system. The one that has had
the best historical success, is the width of the data bus. At present,
this is relatively easy to do. Just count the number of wires
carrying data. As technology advances, this will become harder since
the busses will be on-chip. This means that the actual 64 wires may
not be visible on the circuit board, as they are now. The details may
change, but the basic principal will remain the same.
Now, how many bits is the Jaguar?
The heart of the Jaguar architecture is a 64-bit data bus. This
allows several of the system components to operate on 64-bits of data
simultaneously and pass those results around the same way. These
components are:
=*= Object Processor =*=
This system element takes an image from DRAM (main
memory) to draw it on the screen. It reads DRAM 64-bits
at a time.
=*= BLiTTER =*=
This moves data around the system. It is optimized for
the transfer of graphics data. The BLiTTER is capable of
64-bit reads and writes.
=*= GPU =*=
This is the true computational heart of the system. It
is a custom RISC processor, with a 64-bit data bus, a
32-bit ALU and 32-bit registers.
There are other support components in the Jaguar that are not 64-bit
in nature. This does not prevent the system from being 64-bit.
From this, it should be easier to see that the Atari Jaguar really is
a 64-bit system.
(This document issued at Atari's "Fun 'N' Games" press event held June
20, 1995 at Atari headquarters in Sunnyvale, California. Tradenames
used are Trademarks or Registered Trademarks of their owning
companies.)
PROWLER The Atari Console Disk Magazine February 27, 1996
Copyright (c) 1996 All Rights Reserved Issue No. 01