3 files changed, 63 insertions, 0 deletions
diff --git a/src/arch/x86/cpuid/Makefile b/src/arch/x86/cpuid/Makefile
new file mode 100644
index 0000000..577d96a
--- /dev/null
+++ b/src/arch/x86/cpuid/Makefile
@@ -0,0 +1,5 @@
+a.out: cpuid.c
+	gcc -g cpuid.c
+
+clean:
+	$(RM) a.out
diff --git a/src/arch/x86/cpuid/cpuid.c b/src/arch/x86/cpuid/cpuid.c
new file mode 100644
index 0000000..ab4a7a4
--- /dev/null
+++ b/src/arch/x86/cpuid/cpuid.c
@@ -0,0 +1,47 @@
+#include <stdint.h>
+#include <stdio.h>
+
+typedef struct {
+  uint32_t eax, ebx, ecx, edx;
+} cpuid_t;
+
+// in: eax is the cpuid leaf number
+static inline cpuid_t cpuid(uint32_t eax) {
+  cpuid_t r;
+  asm volatile("cpuid"
+               : "=a"(r.eax), "=b"(r.ebx), "=c"(r.ecx), "=d"(r.edx)
+               : "a"(eax), "c"(0));
+  return r;
+};
+
+// in: eax is the cpuid leaf number
+// in: ecx is the cpuid sub-leaf number
+static inline cpuid_t cpuid_subleaf(uint32_t eax, uint32_t ecx) {
+  cpuid_t r;
+  asm volatile("cpuid"
+               : "=a"(r.eax), "=b"(r.ebx), "=c"(r.ecx), "=d"(r.edx)
+               : "a"(eax), "c"(ecx));
+  return r;
+};
+
+int main() {
+  cpuid_t r;
+  uint32_t leafs = 1;
+
+  for (uint32_t l = 0; l < leafs; ++l) {
+    r = cpuid(l);
+    if (l == 0) {
+      // leaf 0 returns the highest available cpuid leaf in eax.
+      leafs = r.eax;
+
+      // ebx, edx, ecx contain the vendor string
+      // clang-format off
+      printf("vendor: %c%c%c%c%c%c%c%c%c%c%c%c\n",
+             (r.ebx & 0xFF), (r.ebx >> 8) & 0xFF, (r.ebx >> 16) & 0xFF, (r.ebx >> 24) & 0xFF,
+             (r.edx & 0xFF), (r.edx >> 8) & 0xFF, (r.edx >> 16) & 0xFF, (r.edx >> 24) & 0xFF,
+             (r.ecx & 0xFF), (r.ecx >> 8) & 0xFF, (r.ecx >> 16) & 0xFF, (r.ecx >> 24) & 0xFF);
+      // clang-format on
+    }
+    printf("[0x%08x] eax=%08x ebx=%08x ecx=%08x edx=%08x\n", l, r.eax, r.ebx, r.ecx, r.edx);
+  }
+}
diff --git a/src/arch/x86_64.md b/src/arch/x86_64.md
index 56e4f63..0ca18cc 100644
--- a/src/arch/x86_64.md
+++ b/src/arch/x86_64.md
@@ -166,6 +166,17 @@ implemented TSC ticks with a constant frequency.
 grep constant_tsc /proc/cpuinfo
 ```
 
+## Cpu & hw features - `cpuid`
+```x86asm
+cpuid   // in:  eax leaf; ecx sub-leaf
+        // out: eax, ebx, ecx, edx (interpreting depends on leaf)
+```
+
+This instruction is used to query for availability of certain
+instructions or hardware details like cache sizes and son on.
+
+An example how to read cpuid leafs is show in [cpuid.c](x86/cpuid/cpuid.c).
+
 ## [SysV x86_64 ABI][sysvabi]
 
 ### Passing arguments to functions