At 11:55 AM -0600 6/19/2010, Doug McNutt wrote:
Just because nobody seems to have mentioned it. . .
Without multiplexing, 64 bit addressing of external memory requires
64 external pins on the chip. Have you looked at the pins on a
modern processor chip? Where would you put 32 more? How about the
printed circuit wiring that connects the memory address lines to the
bus while keeping all exactly the same electrical length?
Someone has properly chosen fewer than all 64 bits until we really
need those exabytes.
The games played over the years to address this issue have been
interesting. Sometimes there are no separate address lines at all:
they share with the data lines. Sometimes the addressing is done in
two or more blocks - each latched before the data. Ditto for the
data.
Always there are many fewer "lines" than you would expect. That's
because so much of the address space is never mapped, or is mapped to
things other than real memory (so those bits actually point to other
buses). eg: Only 12 to 24 bits on a 32 bit system.
Today's packages (the case in which the chip resides) have some
combination of pins around the edge, pads on the bottom, and pads on
the top. Picture: a bazillion legged spider with a medusa head.
Pretty cool to watch the high speed machines strap the chip into the
package then into circuit board... Just last night I watched one of
those 'how its made' type shows on tv. They showed a chip being
glued directly onto a circuit board then the medusa head being added,
one wire at a time, and then they sealed the whole thing with a big
blob of insulating glue -- no chip package at all!
- Dan.
--
- Psychoceramic Emeritus; South Jersey, USA, Earth.
--
You received this message because you are a member of G-Group, a group for
those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs.
The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette
guide is at http://www.lowendmac.com/lists/netiquette.shtml
To post to this group, send email to g3-5-list@googlegroups.com
For more options, visit this group at http://groups.google.com/group/g3-5-list
