MPU pin

[xxxx]

on the 88088 processor there's a pin named ALE.i know that ALE stands for addres latch enabled.my question is what does it actually do?
what does it do to the address lines?what does latching an address mean?

thanks

[drhowarddrfine]

In most cases the signal is used to enable external registers to "latch" the address lines.  The external registers might actually be inside memory chips.  The purpose is to hold the address steady external to the processor which is now free to change the address lines to something else. 

[xxxx]

how does latching works?

[MichaelW]

This is for the 80286 specifically, but AFAIK it is applicable to all x86 processors.

From ISA System Architecture, Second Edition, Tom Shanley and Don Anderson, Mindshare Press, 1993 ISBN 1-881609-05-7:

Intel dictates that every 80286-based system must incorporate an external device known as an Address Latch. When the microprocessor outputs the address onto its local address bus during a bus cycle, the Bus Control Logic signal ALE (Address Latch Enable) commands the Address Latch to hold (latch) the address and remember it. Once latched, the Address Latch outputs the address to the system on the system address (SA) bus, SA1:SA19. Address decoders throughout the system can then examine the latched address to see if the microprocessor is attempting to communicate with their respective device.

[AeroASM]

a latch is an electronic circuit. It has an in wire and an out wire, and the out wire just shows what the in wire shows. When the latch is activated, the out wire stays as it is, no matter what the in wire shows.

[xxxx]

thanks al for the explanation.my next question is,what is a strobe?

[Randall Hyde]

on the 88088 processor there's a pin named ALE.i know that ALE stands for addres latch enabled.my question is what does it actually do?
what does it do to the address lines?what does latching an address mean?

thanks

On many of the earlier x86 chips, Intel shared various address and data lines on the CPU and time-multiplexed them. The ALE signal told external circuitry that an address could be currently found on the shared lines and an external latch needed to save those values because the shared lines were about to become data lines (and the address would no longer appear on them). You don't find ALE on newer processors because they use separate lines for the address and data bus.
Cheers,
Randy Hyde

[drhowarddrfine]

my next question is,what is a strobe?

Now you're getting beyond the scope of this book, uh, forum.  You need a book to explain the working of processors, ICs and electronics.  A strobe is simply an electronic pulse.  For the latch you talked about it might trigger the latch by just going from positive to zero or vice versa, thus, "strobing" the latch. 

[pbrennick]

If your curious,
Google for J-K flip flops or mono-astable multivibrators.

Paul

[xxxx]

"Now you're getting beyond the scope of this book, uh, forum.  You need a book to explain the working of processors, ICs and electronics.  A strobe is simply an electronic pulse.  For the latch you talked about it might trigger the latch by just going from positive to zero or vice versa, thus, "strobing" the latch. "

the following excerpt is taken from  the book " the 8088 and 8086 microprocessor" this excerpt explains  DRAM decodes an address:

"first the row address is applied to A0 through A7.then RAS is pulsed to logic 0 to latch it into the device.next,the column address is appllied and strobes CAS to logic 0.this 16-bit address selects one of the 64k storage locations to be accesed."

from the excerpt i undestand that CAS is changed from logic 1 to 0.i was just wondering would still be correct to say that CAS is pulsed to
logic 0 instead of strobes to logic 0?and what's the difference?

thanks

[drhowarddrfine]

Yes, they are the same thing.  There is an article in an old Byte magazine describing all this called "Practical Dynamic Memory Design" December 1982 if you can find it.

[dioxin]

xxxx,

what's the difference?

A pulse changes state then changes back again. It may be completely independent of anything else going on.
A strobe is a pulse which is used specifically to time something else or to indicate when something else is ready.

So, a strobe is a special case of a pulse. In the situation you mention, the RAS and CAS lines (Row Address Strobe and Column Address Strobe) the Strobe lines tell the memory chip that the address lines are ready (i.e.have valid data) so that the memory chip can correctly time read/write.

Paul.

[xxxx]

"A pulse changes state then changes back again. It may be completely independent of anything else going on."

you mean 0 to 1 and back to 0 again?or 1 to 0 and back to 1 again?

[dioxin]

xxxx,
   yes, I mean either 0-1-0 or 1-0-1. Both are cloassed as a pulse

Pual.

[xxxx]

thanks a lot.everyone's help is much appreaciated