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• Gpr-indices

Reg-index

Using the REX prefix to access general-purpose registers

Signature

(reg-index reg rex-byte index) → *

Arguments

reg — Register index.

rex-byte — REX prefix.

index — One of the W, R, X, or B bits of the REX prefix.

In 64-bit mode, in addition to generating 64-bit operand sizes, the REX prefix is used to reference registers R8 to R15. Instructions that include REX prefixes can access these registers if the relevant W, R, X, or B bit in the REX prefix is set. E.g., let R be the relevant bit in the REX prefix and let R be set --- so index = 2 for this function. If reg = 0 (which, in the non-REX world, would refer to rAX), reg-index would give us the register index corresponding to the register r8. If R is not set, reg-index will give us the index corresponding to rAX.

In 32-bit mode, the REX prefix is absent. This function can be used in 32-bit mode, by passing 0 as REX.

Definitions and Theorems

Function: reg-index$inline

(defun reg-index$inline (reg rex-byte index)
  (declare (type (unsigned-byte 3) reg)
           (type (unsigned-byte 8) rex-byte)
           (type (unsigned-byte 2) index))
  (if (logbitp index rex-byte)
      (logior 8 (mbe :logic (n03 reg) :exec reg))
    (mbe :logic (n03 reg) :exec reg)))

Theorem: reg-indexp-reg-index

(defthm reg-indexp-reg-index
  (reg-indexp (reg-index reg rex-byte name)
              rex-byte))

Theorem: n04p-reg-index

(defthm n04p-reg-index
 (unsigned-byte-p 4 (reg-index reg rex-byte name))
 :rule-classes
 (:rewrite
  (:type-prescription
      :corollary (natp (reg-index reg rex-byte name))
      :hints
      (("Goal" :in-theory '(unsigned-byte-p integer-range-p natp))))
  (:linear
   :corollary (and (<= 0 (reg-index reg rex-byte name))
                   (< (reg-index reg rex-byte name) 16))
   :hints
   (("Goal"
        :in-theory '(unsigned-byte-p integer-range-p (:e expt)))))))

Theorem: reg-indexp-forward

(defthm reg-indexp-forward
  (implies (reg-indexp reg rex-byte)
           (unsigned-byte-p 4 reg))
  :rule-classes :forward-chaining)