OS/2 FAQ List: User's Edition (4 Apr 94)

(2.6) More Than 16 MB RAM

Can I use more than 16 MB of RAM?

OS/2 2.1 will address all the RAM in your system.  If the BIOS recognizes 
the memory, OS/2 will find it and use it.

However, on certain systems the RAM beyond the 16 MB boundary may be used 
as a fast swap area.  OS/2 relies on its swap file, SWAPPER.DAT, to hold 
code and data which cannot fit into real memory (i.e. to provide virtual 
memory).  If the swap file (and applications) can only be accessed via a 
hard disk adapter which uses 24-bit DMA for disk access (e.g. the Adaptec 
154x series), then the system must move code and data below the 16 MB 
boundary before it can write it to disk.  This "double move" is costly (in 
terms of performance), and often OS/2 will merely use all the RAM above 
the 16 MB boundary as a fast swap area (before writing to disk) to avoid 
the problem.  It is up to the hard disk adapter driver, however, to decide 
how to handle this situation.

Only AT bus adapters are limited to 24-bit DMA.  Microchannel, EISA, VESA 
LocalBus, and other 32-bit adapters are not so limited.  Moreover, only a 
select few AT bus hard disk adapters utilize DMA.  Nearly all MFM, RLL, 
{{ ESDI, }} and IDE adapters, and many SCSI adapters, do not use DMA for 
disk access.

Suffice it to say that, regardless of your present hardware, OS/2 will 
take advantage of it as best it can.  However, if you are planning new 
hardware purchases, you may wish to take this particular hardware design 
limitation into account.  Specifically, if you plan to install more than 
16 MB of RAM in your system, either choose a 32-bit hard disk adapter 
(Microchannel or EISA, for example) or choose an AT bus adapter which does 
not utilize DMA for disk access (a standard IDE adapter, an Adaptec 152x 
series SCSI adapter, or a Future Domain SCSI adapter, for example).  The 
performance trade-off is highly system dependent, however.  You may find 
that even DMA adapters such as the Adaptec 154x series outperform the 
alternatives in certain cases.

Related information:

(2.5) Specific Hardware Recommendations

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