Nifty Memory Tricks

Nifty Memory Tricks

Bill Gates, the erstwhile head of Microsoft
once said, “640 KB ought to be enough for
Banybody!” This statement was made in the
early 90s, when DOS was the de facto PC operating
system. And of course, it lacks foresight. A few years
later the PC hardware industry struggled with the
problem of memory addressability, beyond the 640
KB and 1 MB limits. To overcome that, the industry
resorted to memory tricks, the genius of system
software programmers.
First generation PCs of the mid-80s had
severe memory limitations because of the width
of the address bus. The 8086, a 16-bit system,
had an address bus 20-bits wide, and hence the
maximum addressable memory was 1 MB. But
not all of that precious 1 MB was available to
software applications. In fact, just the fi rst 640 KB
(conventional memory) was up for grabs, on a fi rst
come, fi rst serve basis.
The next 384 KB of estate was called Upper
Memory Area, and reserved for drivers, video RAM,
ROM routines, and other system information.
After loading DOS, BIOS communication
routines, and Terminate and Stay Resident Programs
(TSRs) you had some conventional memory left for
applications. If that application was a hefty one like
Lotus 123 (the de facto spreadsheet in those days),
you’d soon get the “Insuffi cient Memory” system
error message.
There were a few blocks in the upper memory
area that were reserved and unused. These areas
were called Upper Memory Blocks (UMBs). With
some clever programming, certain things like drivers
and TSRs could be shifted to the UMBs, thereby
freeing up conventional memory. The applications
that did such juggling acts were called Memory
Maximizers. The HIMEM.SYS and EMM386.EXE
drivers are prime examples.
As memory boards (see picture) became
available, manufacturers modifi ed the architecture
to accommodate expanded memory. But the 1 MB
memory limit continued to be a pesky problem.
So once again programmers wrote applications to
swap memory pages between expanded memory
(on memory cards) and the UMBs.
Once the 80286 systems came along, memory
addressability was increased to 16 MB. And with th
advent of 32-bit systems like the 80386 and 80486,
you could go all the way to 4 GB (though we are yet
to see PCs with 4 GB memory today). Windows 95
improved memory management, signifi cantly.
Virtual memory paging is another memory trick,
accomplished by the memory manager VMM386.
exe. It was introduced as multitasking came to PCs
(with Windows and 80386 systems); in fact it is still
used today.
I am also awed by other memory shuffl ing tricks
like the DOS Diskcopy utility and Windows Defrag.
Using a single fl oppy drive in the PC, Diskcopy
gave us the freedom to make exact replicas of
fl oppy disks. Depending on the amount of content
on the fl oppy, and the capacity of memory, you had
to keep swapping the master disk and its copy a
number of times in the single drive. With repeated
swapping, the contents from the master would be
copied to another disk via main memory.
The Windows defrag utility also uses main
memory to tidy up fi le storage on the hard disk. It
picks up a cluster of a fragmented fi le and copies it
into main memory. Then it writes that cluster back
to disk in such a manner that all the fi le’s clusters
end up sequentially. And you could see a graphical
rendition of this in the Windows 98 defrag utility.
Going forward, I expect memory density to cross
the 1 Gbit threshold. Then memory will catch up
with hard disks, on the capacity front. When that
happens, you’ll see hybrid systems that use both
Flash memory and hard disk for permanent storage.
Earlier this year, Samsung announced that it
will build hybrid hard disk systems for notebooks.
The Flash memory section will be used for caching
frequently used applications. Eventually, there’ll be
only “solid state disks”.