Editing IBM Personal Computer (section)

=== CPU and RAM ===
[[File:IBM PC Motherboard (1981).jpg|thumb|Original IBM PC motherboard with 16&nbsp;KB RAM soldered and 48&nbsp;KB socketed, for a total of 64&nbsp;KB onboard|alt=]]
[[File:Ibm5150mobo1984black.jpg|thumb|Later IBM PC motherboard with 64&nbsp;KB RAM soldered and 192&nbsp;KB socketed, for a total of 256&nbsp;KB onboard|alt=]]
The CPU is an [[Intel 8088]], a cost-reduced form of the [[Intel 8086]] which largely retains the 8086's internal 16-bit logic, but exposes only an 8-bit bus.<ref name=":0">{{Cite book|url=https://archive.org/stream/byte-magazine-1981-10/1981_10_BYTE_06-10_Local_Networks#page/n31/mode/2up|title=Byte Magazine Volume 06 Number 10 - Local Networks|date=October 1981|pages=28–34}}</ref> The CPU is clocked at 4.77&nbsp;MHz; clones and later PC models have higher CPU speeds that break compatibility with software developed for the original PC.<ref>{{Cite book|last=Graves|first=Michael W.|url=https://books.google.com/books?id=GZFv0Vv8CEcC&q=%22turbo%20button%22%20crash&pg=PA66|title=A+ Guide to PC Hardware Maintenance and Repair|date=September 17, 2004|publisher=Cengage Learning|isbn=978-1-4018-5230-6|language=en}}</ref> The single base clock frequency for the system is 14.31818&nbsp;MHz, which when divided by 3, yielded the 4.77&nbsp;MHz for the CPU (which was considered close enough to the then 5&nbsp;MHz limit of the 8088), and when divided by 4, yields the required 3.579545&nbsp;MHz for the [[NTSC]] color carrier frequency.

The PC motherboard includes a second, empty socket, described by IBM simply as an "auxiliary processor socket", although the most obvious use was the addition of an [[Intel 8087]] math coprocessor, which improves [[Floating-point arithmetic|floating-point math]] performance.<ref>{{Cite book|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n41/mode/2up|title=Byte Magazine Volume 07 Number 01 - The IBM Personal Computer|date=January 1982}}</ref>

PC mainboards were manufactured with the first [[memory bank]] of initially [[Mostek]] 4116-compatible,<ref>[https://minuszerodegrees.net/memory/4116.htm 4116 class RAM] One bank used nine of these chips (eight 16-[[kilobit]] chips = 16 kilobytes, plus one chip for parity).</ref> or later 4164-compatible<ref>[https://minuszerodegrees.net/memory/4164.htm 4164 class RAM] – One bank used nine of these chips (eight 64-kilobit chips = 64 kilobytes, plus one chip for parity).</ref> [[Dual_in-line_package|DIP]] [[DRAM]]s soldered to the board,<ref>{{cite web | url=https://minuszerodegrees.net/5150/ram/5150_ram.htm | title=Minuszerodegrees.net }}</ref> for a minimum configuration of first just 16 KB, or later 64 KB of RAM. Memory upgrades from IBM and third parties provide socketed installation in three further onboard banks, and as [[Industry Standard Architecture|ISA]] expansion cards.

The first 400,000 16&nbsp;KB mainboards ("16KB-64KB" ID) sold until March 1983 can be upgraded to a maximum of 64&nbsp;KB onboard without using slots, and the later 64&nbsp;KB revision ("64KB-256KB" ID) to a maximum of 256&nbsp;KB on the motherboard.<ref name="feldmann19851001">{{Cite magazine |last=Feldmann |first=Peter |date=1985-10-01 |title=Upgrading Your 64K PC |url=https://archive.org/details/PC-Mag-1985-10-01/page/n146/mode/1up?view=theater |access-date=2025-04-13 |magazine=PC |pages=145-149}}</ref> [[RAM card]]s can upgrade either variant further,<ref name=":0" /> for a total of 640 KB [[conventional memory]], and possibly several megabytes of [[expanded memory]] beyond that, though on PC/XT-class machines, the latter was a very expensive [[Expanded memory#Expansion boards|third-party hardware option]] only available later in the IBM 5150's [[product lifecycle|lifecycle]] and only usable with dedicated software support (i.e. only accessible via a RAM window in the [[Upper Memory Area]]<ref>{{cite web|url=https://minuszerodegrees.net/5150/misc/5150%20-%20Memory%20Map%20of%20the%20640%20KB%20to%201%20MB%20Area.jpg|title=IBM 5150 - Memory Map of the 640 KB to 1 MB Area (the 'Upper Memory Area')|website=minuszerodegrees.net|archive-url=https://web.archive.org/web/20250409172823/https://minuszerodegrees.net/5150/misc/5150%20-%20Memory%20Map%20of%20the%20640%20KB%20to%201%20MB%20Area.jpg|archive-date=9 April 2025|url-status=live}}</ref>); this was relatively rarely equipped and utilized on the original IBM PC, much less fully so, thus the machine's maximum RAM configuration as commonly understood is 640&nbsp;KB.