2-4
INSTRUCTION FORMAT
sented by the sixth row, labeled /digit (Opcode). The five rows above give the byte, word, and
doubleword general-purpose registers; the MMX technology registers; the Streaming SIMD
Extensions registers; and SIMD floating-point registers that correspond to the register numbers,
with the same assignments as for the R/M field when Mod field encoding is 11B. As with the
R/M field register options, which of the five possible registers is used is determined by the
opcode byte along with the operand-size attribute.
The body of Tables 2-1 and 2-2 (under the label Value of ModR/M Byte (in Hexadecimal))
contains a 32 by 8 array giving all of the 256 values of the ModR/M byte, in hexadecimal. Bits
3, 4 and 5 are specified by the column of the table in which a byte resides, and the row specifies
bits 0, 1 and 2, and also bits 6 and 7.
