Editing IBM PC XT (5160) (section)

== Memory and ROM ==

The XT has two motherboard revisions, identified by silkscreen marking:

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''IBM 5160 motherboard revisions'''
! Silkscreen marking !! RAM banks !! BIOS shipped !! Notes
|-
| 64-256KB (early) || 4 × 64 KB || 11/08/82 || Also found in the IBM 5155 Portable PC. Can be modified to support 640 KB on-board (IBM-supported procedure).
|-
| 256-640KB (later) || 2 × 256 KB + 2 × 64 KB || 01/10/86 or 05/09/86 || Native support for 640 KB on-board.
|}

The XT motherboard carries the BIOS in two sockets (U18 and U19), unlike the 5150's single BIOS chip in U33. The U19 socket on early-revision boards also contains part of [[IBM Cassette BASIC]]; on later 1986-dated BIOSes, U19 is shared between BIOS and Cassette BASIC.

=== BIOS revisions ===

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''IBM 5160 BIOS revisions'''
! Date !! U18 / U19 part numbers !! ROM type !! Floppy support !! Notes
|-
| 08/16/82 || U18=5000026, U19=5000027 || MK38000 / MK37000 mask ROM || 360 KB || Possibly never publicly released; very rare.
|-
| 11/08/82 || U18=1501512, U19=5000027 (or 6359116) || Mask ROM, varies || 360 KB, partial 720 KB support || Standard first-production BIOS.
|-
| 01/10/86 || U18=62X0851, U19=62X0854 (or U18=62X0852, U19=62X0853) || 27256 EPROM || 360 KB and 720 KB || Adds Enhanced Keyboard support and 1.2 MB drive code (HD floppy controller still required).
|-
| 05/09/86 || U18=62X0890, U19=68X4370 (or 59X7268 / 62X0819) || 27256 EPROM || 360 KB and 720 KB || Final XT BIOS.
|}

The 1986-dated BIOS revisions have two notable caveats: they may produce a specific symptom documented by minuszerodegrees.net, and they may not support an IBM 5161 expansion chassis.

On the 1982-dated BIOS revisions, chip U19 contains part of Cassette BASIC and is not critical to operation. If the POST detects a failed U19, it displays "F6000 ROM" but continues booting. On the 1986-dated BIOS revisions U19 also contains part of the BIOS, so a failed U19 produces a 'dead' motherboard.