IBM 560 - Manual (Page 8)

Attention
The term Reserved describes certain signals, bits, and registers
that should not be changed. Use of reserved areas can cause
compatibility problems, loss of data, or permanent damage to the
hardware. When the contents of a register are changed, the
state of the reserved bits must be preserved. When possible,
read the register first and change only the bits that must be
changed.
viii ThinkPad 560/560E Preface