Hardware Reference
In-Depth Information
6.
Atthispoint,thefirst(orprevious)bankhastestedtobegood,sotheproblemmustbe
in the remaining modules that have been temporarily removed. Install the next bank
of memory and retest. If the problem resurfaces now, the memory in that bank is de-
fective. Continue testing each bank until you find the defective module.
7.
Repeat the preceding step until all remaining banks of memory are installed and have
been tested. If the problem has not resurfaced after you have removed and reinstalled
all the memory, the problem was likely intermittent or caused by poor conduction on
thememorycontacts.Oftensimplyremovingandreplacingmemorycanresolveprob-
lems because of the self-cleaning action between the module and the socket during
removal and reinstallation.
The System Logical Memory Layout
TheoriginalPChadatotalof1MBofaddressablememory,andthetop384KBofthatwas
reservedforusebythesystem.Placingthisreservedspaceatthetop(between640KBand
1,024KB, instead of at the bottom, between 0KB and 640KB) led to what is often called
the
conventional memory barrier
. The constant pressures on system and peripheral man-
ufacturers to maintain compatibility by never breaking from the original memory scheme
of the first PC has resulted in a system memory structure that is (to put it kindly) a mess.
AlmosttwodecadesafterthefirstPCwasintroduced,eventhenewestsystemsarelimited
in many important ways by the memory map of the first PCs.
The original PC used an Intel 8088 processor that could run only 16-bit instructions or
code, which ran in what was called the
real mode
of the processor. These early processors
hadonlyenoughaddresslinestoaccessupto1MBofmemory,andthelast384KBofthat
was reserved for use by the video card as video RAM, other adapters (for on-card ROM
BIOS or RAM buffers), and finally the motherboard ROM BIOS.
The 286 processor brought more address lines, enough to allow up to 16MB of RAM to
be used, and a new mode called protected mode that you had to be in to use. One area
of confusion was that RAM was now noncontiguous; that is, the OS could use the first
640KBandthelast15MB,butnotthe384KBofsystemreservedareathatsatinbetween.
When Intel released the first 32-bit processor in 1985 (the 386DX), the memory archi-
tecture of the system changed dramatically. There were now enough address lines for the
processor to use 4GB of memory, but this was accessible only in 32-bit protected mode,
in which only 32-bit instructions or code could run. Unfortunately, it took 10 years for the
industry to transition from 16-bit to 32-bit OSs and applications. From a software instruc-
tion perspective, all the 32-bit processors since the 386 are really just faster versions of
the same.
