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• Gprs-reads-and-writes

Rr08

Read from byte general-purpose registers

Signature

(rr08 reg rex x86) → *

Source: Intel Manual Volume 1 Section 3.4.1.1 (Jun 2024) (General-Purpose Registers in 64-bit Mode)

In 64-bit mode, there are limitations on accessing byte registers. An instruction cannot reference legacy high-bytes (for example: AH, BH, CH, DH) and one of the new byte registers at the same time (for example: the low byte of the RAX register). However, instructions may reference legacy low-bytes (for example: AL, BL, CL or DL) and new byte registers at the same time (for example: the low byte of the R8 register, or R8L). The architecture enforces this limitation by changing high-byte references (AH, BH, CH, DH) to low byte references (BPL, SPL, DIL, SIL: the low 8 bits for RBP, RSP, RDI and RSI) for instructions using a REX prefix.

In other words, without the REX prefix, indices 0-7 refer to byte registers AL, CL, DL, BL, AH, CH, DH, and BH, whereas with the REX prefix, indices 0-15 refer to AL, CL, DL, BL, SPL, BPL, SIL, DIL, R8L, R9L, R10L, R11L, R12L, R13L, R14L, R15L. This applies to 64-bit mode.

In 32-bit mode, this function is called with 0 as REX.

Definitions and Theorems

Function: rr08$inline

(defun rr08$inline (reg rex x86)
 (declare (xargs :stobjs (x86)))
 (declare (type (unsigned-byte 4) reg)
          (type (unsigned-byte 8) rex))
 (declare (xargs :guard (reg-indexp reg rex)))
 (b* ((reg (mbe :logic (nfix reg) :exec reg)))
  (cond ((or (not (eql rex 0)) (< reg 4))
         (let ((qword (the (signed-byte 64) (rgfi reg x86))))
           (n08 qword)))
        (t (let ((qword (the (signed-byte 64)
                             (rgfi (the (unsigned-byte 4) (- reg 4))
                                   x86))))
             (mbe :logic (part-select qword :low 8 :width 8)
                  :exec (n08 (ash qword -8))))))))

Theorem: n08p-rr08

(defthm n08p-rr08
 (unsigned-byte-p 8 (rr08 reg rex x86))
 :rule-classes
 (:rewrite
  (:type-prescription
      :corollary (natp (rr08 reg rex x86))
      :hints
      (("Goal" :in-theory '(unsigned-byte-p integer-range-p natp))))
  (:linear
   :corollary (and (<= 0 (rr08 reg rex x86))
                   (< (rr08 reg rex x86) 256))
   :hints
   (("Goal"
        :in-theory '(unsigned-byte-p integer-range-p (:e expt)))))))