gpr register size in real/protected mode

3 files changed, 18 insertions, 8 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 55af456..b36e7f6 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -15,12 +15,14 @@
   access size, fixing the prior behavior of reporting no memory access size at all
 * table load/store instructions (lgdt, lidt, lldt, sgdt, sidt, sldt) have correct (mode-dependent)
   memory access sizes, rather than incorrectly varying on operand-size overrides.
-* 64-bit mode: mov seg-to-reg uses 32-bit GPRs for the destination rather than 16-bit.
-  * this is more accurate to the semantic of the instruction, which is why other disassemblers
-    report it this way; for register destinations specifically the segment selector is
-    zero-extended to 64 bits for storage. writing to "eax" in this way implies the 32->64 bit
-    zero-extend, whereas writing to "ax" does not imply any zero-extension. mov reg-to-seg
-    is unchanged and uses a 16-bit form for source GPR.
+* mov seg-to-reg has more careful destination GPR selection.
+  * 64-bit: use 32-bit GPRs for the destination rather than 16-bit.
+    - this is more accurate to the semantic of the instruction, which is why other disassemblers
+      report it this way; for register destinations specifically the segment selector is
+      zero-extended to 64 bits for storage. writing to "eax" in this way implies the 32->64 bit
+      zero-extend, whereas writing to "ax" does not imply any zero-extension. mov reg-to-seg
+      is unchanged and uses a 16-bit form for source GPR.
+  * 32-bit, 16-bit: use 32-bit or 16-bit GPRs as requested by mode and operand size prefixes.
 * fix pextr{b,w,d,q} and extractps first and second operands being backwards.
 * reject 0f0d prefetch/nop with a register operand, which was incorrectly decoded before.
 * maskmovq and maskmovdqu now have their implicit memory access size reported correctly.
diff --git a/src/protected_mode/mod.rs b/src/protected_mode/mod.rs
index 3540aa3..4736352 100644
--- a/src/protected_mode/mod.rs
+++ b/src/protected_mode/mod.rs
@@ -7946,7 +7946,11 @@ fn read_operands<
             instruction.operand_count = 2;
 
             if mem_oper == OperandSpec::RegMMM {
-                instruction.regs[1].bank = RegisterBank::D;
+                if instruction.prefixes.operand_size() {
+                    instruction.regs[1].bank = RegisterBank::W;
+                } else {
+                    instruction.regs[1].bank = RegisterBank::D;
+                }
             } else {
                 instruction.mem_size = 2;
             }
diff --git a/src/real_mode/mod.rs b/src/real_mode/mod.rs
index 715db85..097f6e5 100644
--- a/src/real_mode/mod.rs
+++ b/src/real_mode/mod.rs
@@ -7988,7 +7988,11 @@ fn read_operands<
             instruction.operand_count = 2;
 
             if mem_oper == OperandSpec::RegMMM {
-                instruction.regs[1].bank = RegisterBank::W;
+                if !instruction.prefixes.operand_size() {
+                    instruction.regs[1].bank = RegisterBank::W;
+                } else {
+                    instruction.regs[1].bank = RegisterBank::D;
+                }
             } else {
                 instruction.mem_size = 2;
             }