Intel processors only

UDBGRD

0F 0E
Hidden operands only
Reads from: rAX, rCX
Writes to: rDX, rBX

UDBGWR

0F 0F
Hidden operands only
Reads from: rAX, rCX, (rDX, rBX) / (rDX:rBX)
Writes to: rDX, rBX

Implemented through a new decoder mode

Paper: https://raw.githubusercontent.com/chip-red-pill/udbgInstr/main/paper/undocumented_x86_insts_for_uarch_control.pdf