diff --git a/Makefile b/Makefile
index 4aad448..6ab014a 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ VPATH=$(SRCDIR)
 
 CFLAGS ?= -g
 
-ALL_CFLAGS = -Wall -D_GNU_SOURCE -DARCH=$(ARCH) $(BUILD_INC) $(CFLAGS) $(EXTRA_CFLAGS)
+ALL_CFLAGS = -Wall -D_GNU_SOURCE -DARCH=$(ARCH) -U$(ARCH) $(BUILD_INC) $(CFLAGS) $(EXTRA_CFLAGS)
 
 PROG=risu
 SRCS=risu.c comms.c reginfo.c risu_$(ARCH).c risu_reginfo_$(ARCH).c
@@ -49,6 +49,9 @@ $(PROG): $(OBJS)
 %_$(ARCH).elf: %_$(ARCH).s
 	$(AS) -o $@ $<
 
+%_$(ARCH).elf: %_$(ARCH).S
+	$(CC) $(CPPFLAGS) -o $@ -c $<
+
 clean:
 	rm -f $(PROG) $(OBJS) $(BINS)
 
diff --git a/risu_reginfo_i386.h b/risu_reginfo_i386.h
new file mode 100644
index 0000000..755283a
--- /dev/null
+++ b/risu_reginfo_i386.h
@@ -0,0 +1,35 @@
+/*******************************************************************************
+ * Copyright (c) 2010 Linaro Limited
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     Peter Maydell (Linaro) - initial implementation
+ ******************************************************************************/
+
+#ifndef RISU_REGINFO_I386_H
+#define RISU_REGINFO_I386_H
+
+/*
+ * This is the data structure we pass over the socket.
+ * It is a simplified and reduced subset of what can
+ * be obtained with a ucontext_t*
+ */
+struct reginfo {
+    uint32_t faulting_insn;
+    gregset_t gregs;
+};
+
+/*
+ * For i386, the defines are named REG_EAX, etc.
+ * For x86_64, the defines are name REG_RAX, etc.
+ */
+#ifdef __x86_64__
+# define REG_E(X)   REG_R##X
+#else
+# define REG_E(X)   REG_E##X
+#endif
+
+#endif /* RISU_REGINFO_I386_H */
diff --git a/risu_i386.c b/risu_i386.c
index 5e7e01d..9962b8f 100644
--- a/risu_i386.c
+++ b/risu_i386.c
@@ -14,147 +14,39 @@
 #include <string.h>
 
 #include "risu.h"
-
-/* This is the data structure we pass over the socket.
- * It is a simplified and reduced subset of what can
- * be obtained with a ucontext_t*
- */
-struct reginfo {
-    uint32_t faulting_insn;
-    gregset_t gregs;
-};
-
-#ifndef REG_GS
-/* Assume that either we get all these defines or none */
-#define REG_GS 0
-#define REG_FS 1
-#define REG_ES 2
-#define REG_DS 3
-#define REG_ESP 7
-#define REG_TRAPNO 12
-#define REG_EIP 14
-#define REG_EFL 16
-#define REG_UESP 17
-#endif
-
-struct reginfo master_ri, apprentice_ri;
-
-static int insn_is_ud2(uint32_t insn)
-{
-    return ((insn & 0xffff) == 0x0b0f);
-}
+#include "risu_reginfo_i386.h"
 
 void advance_pc(void *vuc)
 {
-    /* We assume that this is either UD1 or UD2.
-     * This would need tweaking if we want to test
-     * expected undefs on x86.
+    ucontext_t *uc = (ucontext_t *) vuc;
+
+    /*
+     * We assume that this is UD1 as per get_risuop below.
+     * This would need tweaking if we want to test expected undefs.
      */
-    ucontext_t *uc = vuc;
-    uc->uc_mcontext.gregs[REG_EIP] += 2;
+    uc->uc_mcontext.gregs[REG_E(IP)] += 3;
 }
 
-static void fill_reginfo(struct reginfo *ri, ucontext_t * uc)
+void set_ucontext_paramreg(void *vuc, uint64_t value)
 {
-    int i;
-    for (i = 0; i < NGREG; i++) {
-        switch (i) {
-        case REG_ESP:
-        case REG_UESP:
-        case REG_GS:
-        case REG_FS:
-        case REG_ES:
-        case REG_DS:
-        case REG_TRAPNO:
-        case REG_EFL:
-            /* Don't store these registers as it results in mismatches.
-             * In particular valgrind has different values for some
-             * segment registers, and they're boring anyway.
-             * We really shouldn't be ignoring EFL but valgrind doesn't
-             * seem to set it right and I don't care to investigate.
-             */
-            ri->gregs[i] = 0xDEADBEEF;
-            break;
-        case REG_EIP:
-            /* Store the offset from the start of the test image */
-            ri->gregs[i] = uc->uc_mcontext.gregs[i] - image_start_address;
-            break;
-        default:
-            ri->gregs[i] = uc->uc_mcontext.gregs[i];
-            break;
-        }
+    ucontext_t *uc = (ucontext_t *) vuc;
+    uc->uc_mcontext.gregs[REG_E(AX)] = value;
+}
+
+uint64_t get_reginfo_paramreg(struct reginfo *ri)
+{
+    return ri->gregs[REG_E(AX)];
+}
+
+int get_risuop(struct reginfo *ri)
+{
+    if ((ri->faulting_insn & 0xf8ffff) == 0xc0b90f) { /* UD1 %xxx,%eax */
+        return (ri->faulting_insn >> 16) & 7;
     }
-    /* x86 insns aren't 32 bit but we're not really testing x86 so
-     * this is just to distinguish 'do compare' from 'stop'
-     */
-    ri->faulting_insn = *((uint32_t *) uc->uc_mcontext.gregs[REG_EIP]);
+    return -1;
 }
 
-
-int send_register_info(int sock, void *uc)
+uintptr_t get_pc(struct reginfo *ri)
 {
-    struct reginfo ri;
-    fill_reginfo(&ri, uc);
-    return send_data_pkt(sock, &ri, sizeof(ri));
-}
-
-/* Read register info from the socket and compare it with that from the
- * ucontext. Return 0 for match, 1 for end-of-test, 2 for mismatch.
- * NB: called from a signal handler.
- */
-int recv_and_compare_register_info(int sock, void *uc)
-{
-    int resp;
-    fill_reginfo(&master_ri, uc);
-    recv_data_pkt(sock, &apprentice_ri, sizeof(apprentice_ri));
-    if (memcmp(&master_ri, &apprentice_ri, sizeof(master_ri)) != 0) {
-        /* mismatch */
-        resp = 2;
-    } else if (insn_is_ud2(master_ri.faulting_insn)) {
-        /* end of test */
-        resp = 1;
-    } else {
-        /* either successful match or expected undef */
-        resp = 0;
-    }
-    send_response_byte(sock, resp);
-    return resp;
-}
-
-static char *regname[] = {
-    "GS", "FS", "ES", "DS", "EDI", "ESI", "EBP", "ESP",
-    "EBX", "EDX", "ECX", "EAX", "TRAPNO", "ERR", "EIP",
-    "CS", "EFL", "UESP", "SS", 0
-};
-
-static void dump_reginfo(struct reginfo *ri)
-{
-    int i;
-    fprintf(stderr, "  faulting insn %x\n", ri->faulting_insn);
-    for (i = 0; i < NGREG; i++) {
-        fprintf(stderr, "  %s: %x\n", regname[i] ? regname[i] : "???",
-                ri->gregs[i]);
-    }
-}
-
-
-/* Print a useful report on the status of the last comparison
- * done in recv_and_compare_register_info(). This is called on
- * exit, so need not restrict itself to signal-safe functions.
- * Should return 0 if it was a good match (ie end of test)
- * and 1 for a mismatch.
- */
-int report_match_status(void)
-{
-    fprintf(stderr, "match status...\n");
-    fprintf(stderr, "master reginfo:\n");
-    dump_reginfo(&master_ri);
-    fprintf(stderr, "apprentice reginfo:\n");
-    dump_reginfo(&apprentice_ri);
-    if (memcmp(&master_ri, &apprentice_ri, sizeof(master_ri)) == 0) {
-        fprintf(stderr, "match!\n");
-        return 0;
-    }
-    fprintf(stderr, "mismatch!\n");
-    return 1;
+    return ri->gregs[REG_E(IP)];
 }
diff --git a/risu_reginfo_i386.c b/risu_reginfo_i386.c
new file mode 100644
index 0000000..c4dc14a
--- /dev/null
+++ b/risu_reginfo_i386.c
@@ -0,0 +1,153 @@
+/*******************************************************************************
+ * Copyright (c) 2010 Linaro Limited
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     Peter Maydell (Linaro) - initial implementation
+ ******************************************************************************/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ucontext.h>
+#include <assert.h>
+
+#include "risu.h"
+#include "risu_reginfo_i386.h"
+
+const struct option * const arch_long_opts;
+const char * const arch_extra_help;
+
+void process_arch_opt(int opt, const char *arg)
+{
+    abort();
+}
+
+const int reginfo_size(void)
+{
+    return sizeof(struct reginfo);
+}
+
+/* reginfo_init: initialize with a ucontext */
+void reginfo_init(struct reginfo *ri, ucontext_t *uc)
+{
+    int i;
+
+    memset(ri, 0, sizeof(*ri));
+
+    for (i = 0; i < NGREG; i++) {
+        switch (i) {
+        case REG_E(IP):
+            /* Store the offset from the start of the test image.  */
+            ri->gregs[i] = uc->uc_mcontext.gregs[i] - image_start_address;
+            break;
+        case REG_EFL:
+            /* Store only the "flaggy" bits: SF, ZF, AF, PF, CF.  */
+            ri->gregs[i] = uc->uc_mcontext.gregs[i] & 0xd5;
+            break;
+        case REG_E(SP):
+            /* Ignore the stack.  */
+            ri->gregs[i] = 0xdeadbeef;
+            break;
+        case REG_E(AX):
+        case REG_E(BX):
+        case REG_E(CX):
+        case REG_E(DX):
+        case REG_E(DI):
+        case REG_E(SI):
+        case REG_E(BP):
+#ifdef __x86_64__
+        case REG_R8:
+        case REG_R9:
+        case REG_R10:
+        case REG_R11:
+        case REG_R12:
+        case REG_R13:
+        case REG_R14:
+        case REG_R15:
+#endif
+            ri->gregs[i] = uc->uc_mcontext.gregs[i];
+            break;
+        }
+    }
+
+    /*
+     * x86 insns aren't 32 bit but 3 bytes are sufficient to
+     * distinguish 'do compare' from 'stop'.
+     */
+    ri->faulting_insn = *(uint32_t *)uc->uc_mcontext.gregs[REG_E(IP)];
+}
+
+/* reginfo_is_eq: compare the reginfo structs, returns nonzero if equal */
+int reginfo_is_eq(struct reginfo *m, struct reginfo *a)
+{
+    return 0 == memcmp(m, a, sizeof(*m));
+}
+
+static const char *const regname[NGREG] = {
+    [REG_EFL] = "eflags",
+#ifdef __x86_64__
+    [REG_RIP] = "rip",
+    [REG_RAX] = "rax",
+    [REG_RBX] = "rbx",
+    [REG_RCX] = "rcx",
+    [REG_RDX] = "rdx",
+    [REG_RDI] = "rdi",
+    [REG_RSI] = "rsi",
+    [REG_RBP] = "rbp",
+    [REG_RSP] = "rsp",
+    [REG_R8]  = "r8",
+    [REG_R9]  = "r9",
+    [REG_R10] = "r10",
+    [REG_R11] = "r11",
+    [REG_R12] = "r12",
+    [REG_R13] = "r13",
+    [REG_R14] = "r14",
+    [REG_R15] = "r15",
+#else
+    [REG_EIP] = "eip",
+    [REG_EAX] = "eax",
+    [REG_EBX] = "ebx",
+    [REG_ECX] = "ecx",
+    [REG_EDX] = "edx",
+    [REG_EDI] = "edi",
+    [REG_ESI] = "esi",
+    [REG_EBP] = "ebp",
+    [REG_ESP] = "esp",
+#endif
+};
+
+#ifdef __x86_64__
+# define PRIxREG   "%016llx"
+#else
+# define PRIxREG   "%08x"
+#endif
+
+/* reginfo_dump: print state to a stream, returns nonzero on success */
+int reginfo_dump(struct reginfo *ri, FILE *f)
+{
+    int i;
+    fprintf(f, "  faulting insn %x\n", ri->faulting_insn);
+    for (i = 0; i < NGREG; i++) {
+        if (regname[i]) {
+            fprintf(f, "  %-6s: " PRIxREG "\n", regname[i], ri->gregs[i]);
+        }
+    }
+    return !ferror(f);
+}
+
+int reginfo_dump_mismatch(struct reginfo *m, struct reginfo *a, FILE *f)
+{
+    int i;
+    for (i = 0; i < NGREG; i++) {
+        if (m->gregs[i] != a->gregs[i]) {
+            assert(regname[i]);
+            fprintf(f, "Mismatch: %s: " PRIxREG " v " PRIxREG "\n",
+                    regname[i], m->gregs[i], a->gregs[i]);
+        }
+    }
+    return !ferror(f);
+}
diff --git a/configure b/configure
index 65e1819..ca2d7db 100755
--- a/configure
+++ b/configure
@@ -48,12 +48,14 @@ EOF
 }
 
 guess_arch() {
-    if check_define __m68k__ ; then
-        ARCH="m68k"
+    if check_define __aarch64__ ; then
+        ARCH="aarch64"
     elif check_define __arm__ ; then
         ARCH="arm"
-    elif check_define __aarch64__ ; then
-        ARCH="aarch64"
+    elif check_define __i386__ || check_define __x86_64__ ; then
+        ARCH="i386"
+    elif check_define __m68k__ ; then
+        ARCH="m68k"
     elif check_define __powerpc64__ ; then
         ARCH="ppc64"
     else
diff --git a/test_i386.S b/test_i386.S
new file mode 100644
index 0000000..456b99c
--- /dev/null
+++ b/test_i386.S
@@ -0,0 +1,41 @@
+/*#############################################################################
+ * Copyright (c) 2010 Linaro Limited
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     Peter Maydell (Linaro) - initial implementation
+ *###########################################################################*/
+
+/* A trivial test image for x86 */
+
+/* Initialise the registers to avoid spurious mismatches */
+	xor	%eax, %eax
+	sahf				/* init eflags */
+
+	mov	$0x12345678, %eax
+	mov	$0x9abcdef0, %ebx
+	mov	$0x97361234, %ecx
+	mov	$0x84310284, %edx
+	mov	$0x83624173, %edi
+	mov	$0xfaebfaeb, %esi
+	mov	$0x84610123, %ebp
+
+#ifdef __x86_64__
+	movq	$0x123456789abcdef0, %r8
+	movq	$0xaaaabbbbccccdddd, %r9
+	movq	$0x1010101010101010, %r10
+	movq	$0x1111111111111111, %r11
+	movq	$0x1212121212121212, %r12
+	movq	$0x1313131313131313, %r13
+	movq	$0x1414141414141414, %r14
+	movq	$0x1515151515151515, %r15
+#endif
+
+/* do compare */
+	ud1	%eax, %eax
+
+/* exit test */
+	ud1	%ecx, %eax
diff --git a/test_i386.s b/test_i386.s
deleted file mode 100644
index a2140a0..0000000
--- a/test_i386.s
+++ /dev/null
@@ -1,27 +0,0 @@
-;###############################################################################
-;# Copyright (c) 2010 Linaro Limited
-;# All rights reserved. This program and the accompanying materials
-;# are made available under the terms of the Eclipse Public License v1.0
-;# which accompanies this distribution, and is available at
-;# http://www.eclipse.org/legal/epl-v10.html
-;#
-;# Contributors:
-;#     Peter Maydell (Linaro) - initial implementation
-;###############################################################################
-
-; A trivial test image for x86
-
-BITS 32
-; Initialise the registers to avoid spurious mismatches
-mov eax, 0x12345678
-mov ebx, 0x9abcdef0
-mov ecx, 0x97361234
-mov edx, 0x84310284
-mov edi, 0x83624173
-mov esi, 0xfaebfaeb
-mov ebp, 0x84610123
-; UD1 : do compare
-UD1
-
-; UD2 : exit test
-UD2