Editing IBM PC (5150) (section)

== Memory and ROM ==
Two motherboard revisions exist, distinguished by the amount of on-board RAM they accept and clearly marked in the silkscreen near the keyboard port.

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{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''IBM 5150 motherboard revisions'''
! Silkscreen marking !! On-board RAM !! RAM chip type !! Banks
|-
| 16KB-64KB (early) || 16 KB &ndash; 64 KB || 4116 (16 Kbit, requires &minus;5 V) || 4 × 16 KB
|-
| 64KB-256KB (later) || 64 KB &ndash; 256 KB || 4164 (64 Kbit) || 4 × 64 KB
|}

The motherboard carries ROM in five sockets:
* '''U33''' &mdash; BIOS ROM.
* '''U29, U30, U31, U32''' &mdash; [[IBM Cassette BASIC]] (Microsoft BASIC).

There are four documented BIOS revisions, all dated rather than numbered (date format MM/DD/YY):

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''IBM 5150 BIOS revisions'''
! Date !! IBM part number !! Compatible board !! Notes
|-
| 04/24/81 || 5700051 || 16KB-64KB only || Recognises only 544 KB total RAM; ignores BIOS expansion ROMs (no EGA/VGA, no HDD)
|-
| 10/19/81 || 5700671 || 16KB-64KB only || Same two limitations as 04/24/81
|-
| 08/16/82 || 5000024 || (rare/possibly unreleased) || Has a bug where EGA / 5150-compatible VGA fail if SW1 video bits are set ON-ON
|-
| 10/27/82 || 1501476 || 16KB-64KB or 64KB-256KB || Removes the 1981 BIOS limits; bug: a corrupted HDD expansion ROM at C8000 beeps "1 long + 2 short" instead of displaying "C800 ROM"
|}

System configuration is set by '''two banks of DIP switches''' (SW1 and SW2) on the motherboard. The 5150 has no battery-backed CMOS &mdash; the DIP switches are read at every power-up.