What does the /4 mean in FF /4?

x86 assembly opcode machine-code

From the Intel Reference Manual, section "3.1.1.1 Opcode Column in the Instruction Summary Table":

  • /digit — A digit between 0 and 7 indicates that the ModR/M byte of the instruction uses only the r/m (register or memory) operand. The reg field contains the digit that provides an extension to the instruction's opcode.
  • /r — Indicates that the ModR/M byte of the instruction contains a register operand and an r/m operand.

This notation is also usually mentioned in the various descriptions of the ModR/M byte, e.g.

Related

How to use this boolean in an if statement?
Requests library: missing SSL handshake certificates file after cx_Freeze
Mosquitto: Starting in local only mode
GetDIBits and loop through pixels using X, Y
Convert dd/mm/yy and dd/mm/yyyy to Dates
Can't pass mysqli connection in session in php
Subtract a column in a dataframe from many columns in R
JavaFx - Updating GUI
'chcp' is not recognized as an internal or external command, operable program or batch file. on a Windows PC
How to add click event to an element?
Static binding doesn't update when resource changes
'Receiver (<ViewController>) has no segue with identifier 'addSegue'