BOUND - x86-64 instruction reference

boundCheck array index against bounds 
Raises a BR exception when the signed index in the register lies outside the two signed bounds stored in memory.
186 and later·Not available in 64-bit mode
Intel's reference: BOUND 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  bound ,  1632 
Operands reg A general-purpose register matching the operation’s size, for example al, ax, eax or rax. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, which is required by spl, bpl, sil, dil and r8–r15. 
mem A memory location addressed with square brackets, for example [rbx+8] or [label]. 
The first operand is the index register; the second addresses the lower bound followed by the upper bound, each the width of the register. An index below the lower bound or above the upper bound raises a BR exception; an index equal to either bound passes, and the instruction then does nothing.
It exists only in 16- and 32-bit code: 64-bit mode reuses its opcode byte as the EVEX prefix of AVX-512 instructions.