Comment by Tuna-Fish
11 years ago
Note that all of the decimal arithmetic instructions are invalid in 64-bit mode.
They had to scavenge opcode space from somewhere, and the bcd were deemed unnecessary.
11 years ago
Note that all of the decimal arithmetic instructions are invalid in 64-bit mode.
They had to scavenge opcode space from somewhere, and the bcd were deemed unnecessary.
Unfortunately they (AMD) didn't reassign those opcodes for some other purpose - they've just become completely invalid.
Instead, a whole row of useful general-purpose increment and decrement instructions was replaced by 16 REX prefices. A bit odd if you consider that the number of BCD and segment prefix opcodes they made invalid would've been more than enough to be assigned to the new REXes, and still maintain a consistent encoding...