🚀 go-pugleaf

In article <1786@gannet.cl.cam.ac.uk> cet1@cl.cam.ac.uk (C.E. Thompson) writes:
>On IBM 3090s (probably the mainframe that the previous poster had in mind)
>"expanded storage" is RAM which is not directly addressable, but is moved
>in 4Kbyte chunks to and from directly addressable storage by synchronous
>instructions. It is used by MVS and VM as a very-high-speed paging device;
>microcode assists can do the transfers without a software page fault in
>some circumstances.
>
>If someone would like to explain what the economic advantages are to 
>providing extra storage in this form, I would be interested to listen.

Expanded storage is used because it is much faster than disk for paging
operations, and cheaper than mainstore for normal data storage.  On most
mainframes the mainstore is built out of ECC protected static rams.
Expanded storage, is still ECC protected (I think) but is built out of
slower, cheaper dynamic ram.

Current mainframes have not yet found it necessary to implement an entire
2Gigabyte physical mainstore.  Building such an array out of high speed
static rams is still too expensive even for mainframes.  The lower cost
workaround is to provide expanded storage to be used as swap or paging
or data space (at the OS's discretion).  This has proven to be faster
than using disk and cheaper than building all of the memory out of sram.

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Sam Fuller / Amdahl System Performance Architecture

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