IBM 5150 Technical Reference page 103

Address
(Hex)
Mode Function
400-48F ROM BIOS See BIOS Listing
490-4EF Reserved
4FO-4FF Reserved as Intra-Application
Communication Area for any application
500-5FF Reserved for DOS and BASIC
500
DOS
Print Screen Status Flag Store
O-Print Screen Operation Not Active or Successful
Print Screen Operation
1-Print Screen In Progress
255-Error Encountered during Print Screen
Operation
504 DOS
Single Drive Mode Status Byte
510-511 BASIC BASIC's Segment Address Store
512-515 BASIC Clock Interrupt Vector Segment: Offset Store
516-519 BASIC Break Key Interrupt Vector Segment: Offset
Store
51A-51D
BASIC
Disk Error Interrupt Vector Segment: Offset
Store
Reserved Memory Locations
5-10
System
BIOS