In the x86 CPU business it was always Intel who introduced new generations
of CPU's first. After the introduction of the 286, others like Harris,
Fujitsu and AMD followed with clones of Intels design. The same can be
said for the 386 and 486, Intel marketed it first and later others followed
with their clones. When Intel was ready to introduce yet another new generation
of CPU's, the Pentium line, it made sure that no one was allowed to copy
the design. Other manufacturers who had relied on cloning Intel designs
had to design a next generation CPU on their own.
AMD's first try at designing a Pentium class CPU was the K5, the code
name for the core was SSA/5. Designing a CPU from ground up is no easy
task, AMD found that out soon enough. Plagued with design problems, the
K5 was finally introduced on 27 March 1996 at clockspeeds of 75 and 90MHz.
In 1994 Intel had already reached clockspeeds of 90MHz (for a fith generation
CPU), Cyrix managed to reach this speed a year later. AMD finnaly introduced
the K5 at 90 MHz in 1996, by then Intel had the Pentium with clockspeeds
up to 200MHz. At a time when clockspeeds for the mainstream market were
at 133MHz, AMD had a serious problem. Something had to be done if AMD
wanted to stay in business.
The SSA/5 core did not bring what AMD had hoped for in terms of speed
and performance. Even though the SSA/5 core design made use of a highly
efficient RISC core with a CISC instructionset translator, specifications
not found in Intel CPU's until the Pentium Pro! Needless to say that the
core design of the K5 was very much ahead of its time, but it didn't perform
as it should.
Intel had always used CISC for the design of their CPU cores. CISC cores
have a large instruction set which it can execute, this makes the core
design very complicated. RISC on the other hand uses a small instruction
set which makes coredesign less complicated than CISC. When RISC and CISC
are combined, a translator between the instrucion sets is needed. Translating
instructions takes time, so only when the RISC core executes the instructions
very fast and the translator operates very efficient it makes sense to
use this design.
With the poor performing K5 there was nothing else for AMD to do than
to go back to the drawing board and try again, and so they did. Problems
with the SSA/5 core concentrated around the internal cache design and
communication to the L2 cache and system memory. These problems were adressed
and solved in a new K5 core, the 5k86. The new core performed about 20
to 30 percent better than the old one, AMD was back in the race!
The 5K86 core performed very well, so much that at lower clockspeeds a
higher clocked Pentium could be outperformed. To be able to compare the
performance of a K5 with that of a Pentium, AMD used the PR rating. The
K5 PR133 performed like a Pentium 133MHz according to AMD, but the K5's
clockspeed is only 100MHz. While Integer performance was indeed as good
or better than the PR rating suggested, FPU performance was not as strong
but acceptable. The K5 with the new 5k86 core was introduced in October
1996 at speeds of 100, 120 and 133MHz.
1997 AMD introduced the K5 PR166 with a clockspeed of 116,7MHz. The multiplier
for this version is 1.75 x 66MHz bus. No motherboard supported this odd
multiplier, so the motherboard is set to 2.5 x 66MHz and the CPU itself
works at 1.75 x 66MHz. This CPU is the third version of the K5, the cache
allocation was slightly changed in the core design which resulted in another
5 percent performance gain. The PR166 was the last and fastest K5 released
by AMD. A PR200 version was planned and already produced in small quantities,
but with the release of the new K6 near AMD decided not to release a K5
at this speed. The PR200 would have ran at a clockspeed of 133MHz.
The K5 with speeds up to PR133 was designed for use in a Socket 5 mainboard,
later versions could only operate in Socket 7 motherboards. Core voltage
was 3.52v and the size of the core at 0.35 micron is 161 mm2 for the SSA/5
and 181 mm2 for the 5k86.
Looking back at 1996 and the situation that AMD was in, Intel with their
Pentium already steady on the market, Cyrix having the 6x86 and doing
reasonably well with it. If AMD wanted to stay in the race they had to
do something. AMD had no real other choice than to release the K5, even
when they knew it was not ready yet. The SSA/5 K5 was just not ready for
Because of the troubled first version of the K5, sales were low and could
not gain momentum. A deal with Compaq to use the K5 as soon as it was
introduced fell apart when it could not meet expectations. Only at the
end of the production cycle Acer and Hewlet Packerd used the K5 in their
PC's. AMD seiced production of the K5 at the end of Q2 1997.
Available models :
AMD K5 PR75 (SSA/5), 75MHz, 1.5 x 50MHz
AMD K5 PR90 (SSA/5), 90MHz, 1.5 x 60MHz
AMD K5 PR100 (SSA/5), 100MHz, 1.5 x 66MHz
AMD K5 PR100 (5k86), 75MHz, 1.5 x 50MHz
AMD K5 PR120 (5k86), 90MHz, 1.5 x 60MHz
AMD K5 PR133 (5k86), 100MHz, 1.5 x 66MHz
AMD K5 PR150 (5k86), 105MHz, 1.75 x 60MHz
AMD K5 PR166 (5k86), 116.5MHz, 1.75 x 66MHz (External 2.5 x 66MHz)
AMD K5 pictures