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            • 32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode
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• Prefix-modrm-sib-decoding

Mandatory-prefixes-computation

Picking a mandatory prefix

When should we interpret a SIMD prefix (66, F2, F3) as the mandatory prefix for a given opcode in the two- and three-byte opcode maps?

Here are some decoding rules for SIMD mandatory prefixes; note that these do NOT apply for VEX/EVEX-encoded instructions because the mandatory prefixes are explicitly stated there. All the examples listed below are from Intel's XED (x86 Encoder Decoder: https://intelxed.github.io/).

1. For opcodes that can take mandatory prefixes, 66 is ignored when F2/F3 are present. Also, a mandatory prefix does not have to immediately precede the opcode byte --- see (4) below.

   Examples:

   (1) xed -64 -d   660f6f00     ;; movdqa xmm0, xmmword ptr [rax] 
   (2) xed -64 -d   f30f6f00     ;; movdqu xmm0, xmmword ptr [rax] 
   (3) xed -64 -d 66f30f6f00     ;; movdqu xmm0, xmmword ptr [rax] (same as (2)) 
   (4) xed -64 -d f3660f6f00     ;; movdqu xmm0, xmmword ptr [rax] (same as (2)) 
    
   

2. For opcodes that can take mandatory prefixes, the presence of an unsupported SIMD prefix translates to a reserved instruction; such a prefix does NOT act as a modifier prefix.

   Examples: Opcode 0f 6b has a no-prefix form and 66 mandatory prefix form. When used with f3, it leads to an error; see (3) below.

   (1) xed -64 -d     0f6b00     ;; packssdw mmx0, qword ptr [rax] 
   (2) xed -64 -d   660f6b00     ;; packssdw xmm0, xmmword ptr [rax] 
   (3) xed -64 -d f3660f6b00     ;; GENERAL_ERROR Could not decode... 
    
   

Definitions and Theorems

Function: 64-bit-compute-mandatory-prefix-for-two-byte-opcode$inline

(defun 64-bit-compute-mandatory-prefix-for-two-byte-opcode$inline
       (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let ((__function__
            '64-bit-compute-mandatory-prefix-for-two-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '64-bit-mode-two-byte-f3-ok
                            *64-bit-mode-two-byte-f3-ok-ar* opcode))
                (and (equal rep-pfx 242)
                     (aref1 '64-bit-mode-two-byte-f2-ok
                            *64-bit-mode-two-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '64-bit-mode-two-byte-66-ok
                       *64-bit-mode-two-byte-66-ok-ar* opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-64-bit-compute-mandatory-prefix-for-two-byte-opcode

(defthm
 return-type-of-64-bit-compute-mandatory-prefix-for-two-byte-opcode
 (b* ((mandatory-prefix (64-bit-compute-mandatory-prefix-for-two-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: 32-bit-compute-mandatory-prefix-for-two-byte-opcode$inline

(defun 32-bit-compute-mandatory-prefix-for-two-byte-opcode$inline
       (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let ((__function__
            '32-bit-compute-mandatory-prefix-for-two-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '32-bit-mode-two-byte-f3-ok
                            *32-bit-mode-two-byte-f3-ok-ar* opcode))
                (and (equal rep-pfx 242)
                     (aref1 '32-bit-mode-two-byte-f2-ok
                            *32-bit-mode-two-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '32-bit-mode-two-byte-66-ok
                       *32-bit-mode-two-byte-66-ok-ar* opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-32-bit-compute-mandatory-prefix-for-two-byte-opcode

(defthm
 return-type-of-32-bit-compute-mandatory-prefix-for-two-byte-opcode
 (b* ((mandatory-prefix (32-bit-compute-mandatory-prefix-for-two-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: 64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline

(defun
 64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
 (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let
  ((__function__
      '64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '64-bit-mode-0f-38-three-byte-f3-ok
                            *64-bit-mode-0f-38-three-byte-f3-ok-ar*
                            opcode))
                (and (equal rep-pfx 242)
                     (aref1 '64-bit-mode-0f-38-three-byte-f2-ok
                            *64-bit-mode-0f-38-three-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '64-bit-mode-0f-38-three-byte-66-ok
                       *64-bit-mode-0f-38-three-byte-66-ok-ar*
                       opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode

(defthm
 return-type-of-64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode
 (b* ((mandatory-prefix (64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: 32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline

(defun
 32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
 (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let
  ((__function__
      '32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '32-bit-mode-0f-38-three-byte-f3-ok
                            *32-bit-mode-0f-38-three-byte-f3-ok-ar*
                            opcode))
                (and (equal rep-pfx 242)
                     (aref1 '32-bit-mode-0f-38-three-byte-f2-ok
                            *32-bit-mode-0f-38-three-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '32-bit-mode-0f-38-three-byte-66-ok
                       *32-bit-mode-0f-38-three-byte-66-ok-ar*
                       opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode

(defthm
 return-type-of-32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode
 (b* ((mandatory-prefix (32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: 64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline

(defun
 64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
 (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let
  ((__function__
      '64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '64-bit-mode-0f-3a-three-byte-f3-ok
                            *64-bit-mode-0f-3a-three-byte-f3-ok-ar*
                            opcode))
                (and (equal rep-pfx 242)
                     (aref1 '64-bit-mode-0f-3a-three-byte-f2-ok
                            *64-bit-mode-0f-3a-three-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '64-bit-mode-0f-3a-three-byte-66-ok
                       *64-bit-mode-0f-3a-three-byte-66-ok-ar*
                       opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode

(defthm
 return-type-of-64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode
 (b* ((mandatory-prefix (64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: 32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline

(defun
 32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
 (opcode prefixes)
 (declare (type (unsigned-byte 8) opcode)
          (type (unsigned-byte 52) prefixes))
 (declare (xargs :guard (prefixes-p prefixes)))
 (let
  ((__function__
      '32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode))
  (declare (ignorable __function__))
  (let ((rep-pfx (the (unsigned-byte 8)
                      (prefixes->rep prefixes))))
    (if (not (eql rep-pfx 0))
        (if (or (and (equal rep-pfx 243)
                     (aref1 '32-bit-mode-0f-3a-three-byte-f3-ok
                            *32-bit-mode-0f-3a-three-byte-f3-ok-ar*
                            opcode))
                (and (equal rep-pfx 242)
                     (aref1 '32-bit-mode-0f-3a-three-byte-f2-ok
                            *32-bit-mode-0f-3a-three-byte-f2-ok-ar*
                            opcode)))
            rep-pfx
          0)
      (let ((opr-pfx (the (unsigned-byte 8)
                          (prefixes->opr prefixes))))
        (if (not (eql opr-pfx 0))
            (if (aref1 '32-bit-mode-0f-3a-three-byte-66-ok
                       *32-bit-mode-0f-3a-three-byte-66-ok-ar*
                       opcode)
                opr-pfx
              0)
          0))))))

Theorem: return-type-of-32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode

(defthm
 return-type-of-32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode
 (b* ((mandatory-prefix (32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
                             opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: compute-mandatory-prefix-for-two-byte-opcode$inline

(defun compute-mandatory-prefix-for-two-byte-opcode$inline
       (proc-mode opcode prefixes)
  (declare (type (integer 0 4) proc-mode)
           (type (unsigned-byte 8) opcode)
           (type (unsigned-byte 52) prefixes))
  (case proc-mode
    (0 (64-bit-compute-mandatory-prefix-for-two-byte-opcode
            opcode prefixes))
    (otherwise (32-bit-compute-mandatory-prefix-for-two-byte-opcode
                    opcode prefixes))))

Theorem: return-type-of-compute-mandatory-prefix-for-two-byte-opcode

(defthm return-type-of-compute-mandatory-prefix-for-two-byte-opcode
  (b* ((mandatory-prefix (compute-mandatory-prefix-for-two-byte-opcode$inline
                              proc-mode opcode prefixes)))
    (unsigned-byte-p 8 mandatory-prefix))
  :rule-classes :rewrite)

Function: compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline

(defun compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
       (proc-mode opcode prefixes)
  (declare (type (integer 0 4) proc-mode)
           (type (unsigned-byte 8) opcode)
           (type (unsigned-byte 52) prefixes))
  (case proc-mode
    (0 (64-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode
            opcode prefixes))
    (otherwise (32-bit-compute-mandatory-prefix-for-0f-38-three-byte-opcode
                    opcode prefixes))))

Theorem: return-type-of-compute-mandatory-prefix-for-0f-38-three-byte-opcode

(defthm
 return-type-of-compute-mandatory-prefix-for-0f-38-three-byte-opcode
 (b* ((mandatory-prefix (compute-mandatory-prefix-for-0f-38-three-byte-opcode$inline
                             proc-mode opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline

(defun compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
       (proc-mode opcode prefixes)
  (declare (type (integer 0 4) proc-mode)
           (type (unsigned-byte 8) opcode)
           (type (unsigned-byte 52) prefixes))
  (case proc-mode
    (0 (64-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode
            opcode prefixes))
    (otherwise (32-bit-compute-mandatory-prefix-for-0f-3a-three-byte-opcode
                    opcode prefixes))))

Theorem: return-type-of-compute-mandatory-prefix-for-0f-3a-three-byte-opcode

(defthm
 return-type-of-compute-mandatory-prefix-for-0f-3a-three-byte-opcode
 (b* ((mandatory-prefix (compute-mandatory-prefix-for-0f-3a-three-byte-opcode$inline
                             proc-mode opcode prefixes)))
   (unsigned-byte-p 8 mandatory-prefix))
 :rule-classes :rewrite)

Function: compute-mandatory-prefix-for-three-byte-opcode$inline

(defun compute-mandatory-prefix-for-three-byte-opcode$inline
       (proc-mode second-escape-byte opcode prefixes)
  (declare (type (integer 0 4) proc-mode)
           (type (unsigned-byte 8)
                 second-escape-byte)
           (type (unsigned-byte 8) opcode)
           (type (unsigned-byte 52) prefixes))
  (declare (xargs :guard (or (equal second-escape-byte 56)
                             (equal second-escape-byte 58))))
  (case second-escape-byte
    (56 (compute-mandatory-prefix-for-0f-38-three-byte-opcode
             proc-mode opcode prefixes))
    (58 (compute-mandatory-prefix-for-0f-3a-three-byte-opcode
             proc-mode opcode prefixes))
    (otherwise 0)))

Theorem: return-type-of-compute-mandatory-prefix-for-three-byte-opcode

(defthm
      return-type-of-compute-mandatory-prefix-for-three-byte-opcode
  (b* ((mandatory-prefix (compute-mandatory-prefix-for-three-byte-opcode$inline
                              proc-mode
                              second-escape-byte opcode prefixes)))
    (unsigned-byte-p 8 mandatory-prefix))
  :rule-classes :rewrite)