Index: lldb/test/Shell/Register/Inputs/x86-fp-read.cpp
===================================================================
--- /dev/null
+++ lldb/test/Shell/Register/Inputs/x86-fp-read.cpp
@@ -0,0 +1,43 @@
+#include <cstdint>
+
+struct alignas(16) float80_raw {
+  uint64_t mantissa;
+  uint16_t sign_exp;
+};
+
+int main() {
+  float80_raw st[] = {
+    {0x8000000000000000, 0x4000},  // +2.0
+    {0x3f00000000000000, 0x0000},  // 1.654785e-4932 (denormal)
+    {0x0000000000000000, 0x0000},  // +0
+    {0x0000000000000000, 0x8000},  // -0
+    {0x8000000000000000, 0x7fff},  // +inf
+    {0x8000000000000000, 0xffff},  // -inf
+    {0xc000000000000000, 0xffff},  // nan
+    // leave st7 empty to test tag word better
+  };
+
+  // unmask invalid op exception
+  uint16_t cw = 0x037e;
+
+  asm volatile(
+    "finit\n\t"
+    "fldcw %1\n\t"
+    // load on stack in reverse order to make the result easier to read
+    "fldt 0x60(%0)\n\t"
+    "fldt 0x50(%0)\n\t"
+    "fldt 0x40(%0)\n\t"
+    "fldt 0x30(%0)\n\t"
+    "fldt 0x20(%0)\n\t"
+    "fldt 0x10(%0)\n\t"
+    "fldt 0x00(%0)\n\t"
+    // this should trigger a stack fault
+    "fdiv %%st(0), %%st(7)\n\t"
+    "int3\n\t"
+    :
+    : "a"(st), "m"(cw)
+    : "st"
+  );
+
+  return 0;
+}
Index: lldb/test/Shell/Register/Inputs/x86-fp-write.cpp
===================================================================
--- /dev/null
+++ lldb/test/Shell/Register/Inputs/x86-fp-write.cpp
@@ -0,0 +1,41 @@
+#include <cstdint>
+#include <cstdio>
+
+struct alignas(16) float80_raw {
+  uint8_t data[10];
+};
+
+int main() {
+  float80_raw st[8];
+  uint16_t env[14];
+
+  asm volatile(
+    "finit\n\t"
+    "int3\n\t"
+    "fstenv %1\n\t"
+    "fstpt 0x00(%0)\n\t"
+    "fstpt 0x10(%0)\n\t"
+    "fstpt 0x20(%0)\n\t"
+    "fstpt 0x30(%0)\n\t"
+    "fstpt 0x40(%0)\n\t"
+    "fstpt 0x50(%0)\n\t"
+    "fstpt 0x60(%0)\n\t"
+    "fstpt 0x70(%0)\n\t"
+    :
+    : "a"(st), "m"(env)
+    : "st"
+  );
+
+  printf("fctrl = 0x%04x\n", env[0]);
+  printf("fstat = 0x%04x\n", env[2]);
+  printf("ftag = 0x%04x\n", env[4]);
+
+  for (int i = 0; i < 8; ++i) {
+    printf("st%d = { ", i);
+    for (int j = 0; j < sizeof(st->data); ++j)
+      printf("0x%02x ", st[i].data[j]);
+    printf("}\n");
+  }
+
+  return 0;
+}
Index: lldb/test/Shell/Register/x86-fp-read.test
===================================================================
--- /dev/null
+++ lldb/test/Shell/Register/x86-fp-read.test
@@ -0,0 +1,24 @@
+# REQUIRES: native && (target-x86 || target-x86_64)
+# RUN: %clangxx_host %p/Inputs/x86-fp-read.cpp -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+process launch
+
+register read --all
+# CHECK-DAG: fctrl = 0x037e
+# CHECK-DAG: fstat = 0x88c1
+# TODO: the following value is incorrect, it's a bug in the way
+# FXSAVE/XSAVE is interpreted; it should be 0xa963 once fixed
+# CHECK-DAG: ftag = 0x00fe
+# CHECK-DAG: fop = 0x04f7
+
+# CHECK-DAG: st0 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0x00 0x40}
+# CHECK-DAG: st1 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x3f 0x00 0x00}
+# CHECK-DAG: st2 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}
+# CHECK-DAG: st3 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80}
+# CHECK-DAG: st4 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0x7f}
+# CHECK-DAG: st5 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0xff}
+# CHECK-DAG: st6 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xc0 0xff 0xff}
+# CHECK-DAG: st7 = {0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}
+
+process continue
+# CHECK: Process {{[0-9]+}} exited with status = 0
Index: lldb/test/Shell/Register/x86-fp-write.test
===================================================================
--- /dev/null
+++ lldb/test/Shell/Register/x86-fp-write.test
@@ -0,0 +1,35 @@
+# REQUIRES: native && (target-x86 || target-x86_64)
+# RUN: %clangxx_host %p/Inputs/x86-fp-write.cpp -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+process launch
+
+register write fctrl 0x037f
+register write fstat 0x0841
+# note: this needs to enable all registers for writes to be effective
+# TODO: fix it to use proper ftag values instead of 'abridged'
+register write ftag 0x00ff
+register write st0 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0x00 0x40}"
+register write st1 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x3f 0x00 0x00}"
+register write st2 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}"
+register write st3 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80}"
+register write st4 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0x7f}"
+register write st5 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0xff}"
+register write st6 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xc0 0xff 0xff}"
+register write st7 "{0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00}"
+
+process continue
+
+# CHECK-DAG: fctrl = 0x037f
+# CHECK-DAG: fstat = 0x0841
+# CHECK-DAG: ftag = 0xa961
+
+# CHECK-DAG: st0 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0x00 0x40 }
+# CHECK-DAG: st1 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x3f 0x00 0x00 }
+# CHECK-DAG: st2 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 }
+# CHECK-DAG: st3 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 }
+# CHECK-DAG: st4 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0x7f }
+# CHECK-DAG: st5 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0xff 0xff }
+# CHECK-DAG: st6 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xc0 0xff 0xff }
+# CHECK-DAG: st7 = { 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 }
+
+# CHECK: Process {{[0-9]+}} exited with status = 0