diff --git a/src/xenia/cpu/backend/a64/a64_backend.cc b/src/xenia/cpu/backend/a64/a64_backend.cc
index 211ddc97d..e6ebc045d 100644
--- a/src/xenia/cpu/backend/a64/a64_backend.cc
+++ b/src/xenia/cpu/backend/a64/a64_backend.cc
@@ -49,8 +49,17 @@ class A64ThunkEmitter : public A64Emitter {
   // The following four functions provide save/load functionality for registers.
   // They assume at least StackLayout::THUNK_STACK_SIZE bytes have been
   // allocated on the stack.
+
+  // Caller saved:
+  // Dont assume these registers will survive a subroutine call
+  // x0, v0 is not saved/preserved since this is used to return values from
+  // subroutines x1-x15, x30 | d0-d7 and d16-v31
   void EmitSaveVolatileRegs();
   void EmitLoadVolatileRegs();
+
+  // Callee saved:
+  // Subroutines must preserve these registers if they intend to use them
+  // x19-x30 | d8-d15
   void EmitSaveNonvolatileRegs();
   void EmitLoadNonvolatileRegs();
 };
@@ -78,7 +87,7 @@ bool A64Backend::Initialize(Processor* processor) {
   std::strcpy(fprs.name, "v");
   fprs.types = MachineInfo::RegisterSet::FLOAT_TYPES |
                MachineInfo::RegisterSet::VEC_TYPES;
-  fprs.count = A64Emitter::XMM_COUNT;
+  fprs.count = A64Emitter::FPR_COUNT;
 
   code_cache_ = A64CodeCache::Create();
   Backend::code_cache_ = code_cache_.get();
diff --git a/src/xenia/cpu/backend/a64/a64_emitter.cc b/src/xenia/cpu/backend/a64/a64_emitter.cc
index 744e2548a..9325e5f3c 100644
--- a/src/xenia/cpu/backend/a64/a64_emitter.cc
+++ b/src/xenia/cpu/backend/a64/a64_emitter.cc
@@ -62,12 +62,13 @@ static const size_t kMaxCodeSize = 1_MiB;
 static const size_t kStashOffset = 32;
 // static const size_t kStashOffsetHigh = 32 + 32;
 
-const uint32_t A64Emitter::gpr_reg_map_[A64Emitter::GPR_COUNT] = {
-    1, 10, 11, 12, 13, 14, 15,
+// Register indices that the HIR is allowed to use for operands
+const uint8_t A64Emitter::gpr_reg_map_[A64Emitter::GPR_COUNT] = {
+    19, 20, 21, 22, 23, 24, 25, 26,
 };
 
-const uint32_t A64Emitter::xmm_reg_map_[A64Emitter::XMM_COUNT] = {
-    4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
+const uint8_t A64Emitter::fpr_reg_map_[A64Emitter::FPR_COUNT] = {
+    8, 9, 10, 11, 12, 13, 14, 15,
 };
 
 A64Emitter::A64Emitter(A64Backend* backend)
@@ -665,8 +666,8 @@ oaknut::XReg A64Emitter::GetNativeParam(uint32_t param) {
 }
 
 // Important: If you change these, you must update the thunks in a64_backend.cc!
-oaknut::XReg A64Emitter::GetContextReg() { return X19; }
-oaknut::XReg A64Emitter::GetMembaseReg() { return X20; }
+oaknut::XReg A64Emitter::GetContextReg() { return X27; }
+oaknut::XReg A64Emitter::GetMembaseReg() { return X28; }
 
 void A64Emitter::ReloadContext() {
   // mov(GetContextReg(), qword[rsp + StackLayout::GUEST_CTX_HOME]);
diff --git a/src/xenia/cpu/backend/a64/a64_emitter.h b/src/xenia/cpu/backend/a64/a64_emitter.h
index d44700847..60641fdba 100644
--- a/src/xenia/cpu/backend/a64/a64_emitter.h
+++ b/src/xenia/cpu/backend/a64/a64_emitter.h
@@ -137,13 +137,13 @@ class A64Emitter : public oaknut::CodeBlock, public oaknut::CodeGenerator {
             std::vector<SourceMapEntry>* out_source_map);
 
  public:
-  // Reserved:  XSP, X19, X20
-  // Scratch:   X0/X1/X2
+  // Reserved:  XSP, X27, X28
+  // Scratch:   X1-X15, X30 | V0-v7 and V16-V31
   //            V0-2
-  // Available: X1, X10-r15
+  // Available: X19-X26
   //            V4-V15 (save to get V3)
-  static const int GPR_COUNT = 7;
-  static const int XMM_COUNT = 12;
+  static const size_t GPR_COUNT = 8;
+  static const size_t FPR_COUNT = 8;
 
   static void SetupReg(const hir::Value* v, oaknut::WReg& r) {
     const auto idx = gpr_reg_map_[v->reg.index];
@@ -154,15 +154,15 @@ class A64Emitter : public oaknut::CodeBlock, public oaknut::CodeGenerator {
     r = oaknut::XReg(idx);
   }
   static void SetupReg(const hir::Value* v, oaknut::SReg& r) {
-    const auto idx = xmm_reg_map_[v->reg.index];
+    const auto idx = fpr_reg_map_[v->reg.index];
     r = oaknut::SReg(idx);
   }
   static void SetupReg(const hir::Value* v, oaknut::DReg& r) {
-    const auto idx = xmm_reg_map_[v->reg.index];
+    const auto idx = fpr_reg_map_[v->reg.index];
     r = oaknut::DReg(idx);
   }
   static void SetupReg(const hir::Value* v, oaknut::QReg& r) {
-    const auto idx = xmm_reg_map_[v->reg.index];
+    const auto idx = fpr_reg_map_[v->reg.index];
     r = oaknut::QReg(idx);
   }
 
@@ -247,8 +247,8 @@ class A64Emitter : public oaknut::CodeBlock, public oaknut::CodeGenerator {
 
   size_t stack_size_ = 0;
 
-  static const uint32_t gpr_reg_map_[GPR_COUNT];
-  static const uint32_t xmm_reg_map_[XMM_COUNT];
+  static const uint8_t gpr_reg_map_[GPR_COUNT];
+  static const uint8_t fpr_reg_map_[FPR_COUNT];
 };
 
 }  // namespace a64