From 6232e8b1daf7067cb2e8065687530d5f88ecb46d Mon Sep 17 00:00:00 2001
From: iximeow <me@iximeow.net>
Date: Fri, 15 Jan 2021 18:40:31 -0800
Subject: support xchg AX/reg

---
 CHANGELOG                  |  1 +
 src/long_mode/mod.rs       | 14 ++++++++++++++
 src/protected_mode/mod.rs  | 11 +++++++++++
 test/long_mode/mod.rs      |  8 ++++++++
 test/protected_mode/mod.rs |  7 +++++++
 5 files changed, 41 insertions(+)

diff --git a/CHANGELOG b/CHANGELOG
index c608941..0a707eb 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -5,6 +5,7 @@
   - `rdpkru` and `wrpkru` were incorrectly decoded when modrm bits were not `11`
 * small performance tweaks. read_imm_signed is now inline(always) and some
   pre-decode initialization is a bit better-packed
+* `xchg {r,e,}ax, reg` was not supported! it's supported now.
 
 ## 0.1.4
 * [long mode only]: fix decoding of rex-prefixed modrm+sib operands selecting index 0b100 and base 0b101
diff --git a/src/long_mode/mod.rs b/src/long_mode/mod.rs
index f9be9ab..909157b 100644
--- a/src/long_mode/mod.rs
+++ b/src/long_mode/mod.rs
@@ -6079,6 +6079,20 @@ fn read_operands<T: Iterator<Item=u8>>(decoder: &InstDecoder, mut bytes_iter: T,
                     }
                     1 => {
                         // Zv_AX
+                        let opwidth = imm_width_from_prefixes_64(SizeCode::vqp, instruction.prefixes);
+                        let bank = if opwidth == 4 {
+                            RegisterBank::D
+                        } else if opwidth == 2 {
+                            RegisterBank::W
+                        } else {
+                            RegisterBank::Q
+                        };
+                        instruction.modrm_rrr =
+                            RegSpec::from_parts(0, instruction.prefixes.rex().b(), bank);
+                        instruction.operands[1] = OperandSpec::RegMMM;
+                        instruction.modrm_mmm =
+                            RegSpec::from_parts(reg, instruction.prefixes.rex().b(), bank);
+                        instruction.operand_count = 2;
                     }
                     2 => {
                         // these are Zb_Ib_R
diff --git a/src/protected_mode/mod.rs b/src/protected_mode/mod.rs
index 9327f64..6394de0 100644
--- a/src/protected_mode/mod.rs
+++ b/src/protected_mode/mod.rs
@@ -5933,6 +5933,17 @@ fn read_operands<T: Iterator<Item=u8>>(decoder: &InstDecoder, mut bytes_iter: T,
                     }
                     1 => {
                         // Zv_AX
+                        let bank = if !instruction.prefixes.operand_size() {
+                            RegisterBank::D
+                        } else {
+                            RegisterBank::W
+                        };
+                        instruction.modrm_rrr =
+                            RegSpec::from_parts(0, bank);
+                        instruction.operands[1] = OperandSpec::RegMMM;
+                        instruction.modrm_mmm =
+                            RegSpec::from_parts(reg, bank);
+                        instruction.operand_count = 2;
                     }
                     2 => {
                         // these are Zb_Ib_R
diff --git a/test/long_mode/mod.rs b/test/long_mode/mod.rs
index 0830c2d..c3933ab 100644
--- a/test/long_mode/mod.rs
+++ b/test/long_mode/mod.rs
@@ -1137,6 +1137,14 @@ fn test_mov() {
 }
 
 #[test]
+fn test_xchg() {
+    test_display(&[0x90], "nop");
+    test_display(&[0x91], "xchg eax, ecx");
+    test_display(&[0x4f, 0x91], "xchg r8, r9");
+    test_display(&[0x66, 0x91], "xchg ax, cx");
+}
+
+#[test]
 fn test_stack() {
     test_display(&[0x66, 0x41, 0x50], "push r8w");
 }
diff --git a/test/protected_mode/mod.rs b/test/protected_mode/mod.rs
index e3c7975..88cf7ea 100644
--- a/test/protected_mode/mod.rs
+++ b/test/protected_mode/mod.rs
@@ -1030,6 +1030,13 @@ fn test_mov() {
 }
 
 #[test]
+fn test_xchg() {
+    test_display(&[0x90], "nop");
+    test_display(&[0x91], "xchg eax, ecx");
+    test_display(&[0x66, 0x91], "xchg ax, cx");
+}
+
+#[test]
 fn test_stack() {
     test_display(&[0x66, 0x50], "push ax");
 }
-- 
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