target-i386/translate.c: mov to/from crN/drN: ignore mod bits

Matthew Ogilvie · malc · commit 5c73b757e3aa · 2012-08-24T07:44:37.000+04:00
&gt; This instruction is always treated as a register-to-register (MOD = 11)
&gt; instruction, regardless of the encoding of the MOD field in the MODR/M
&gt; byte.

Also, Microport UNIX System V/386 v 2.1 (ca 1987) runs fine on
real Intel 386 and 486 CPU's (at least), but does not run in qemu without
this patch.

Signed-off-by: Matthew Ogilvie &lt;mmogilvi_qemu@miniinfo.net&gt;
Signed-off-by: malc &lt;av1474@comtv.ru&gt;
diff --git a/target-i386/translate.c b/target-i386/translate.c
@@ -7551,8 +7551,11 @@ static target_ulong disas_insn(DisasContext *s, target_ulong pc_start)
             gen_exception(s, EXCP0D_GPF, pc_start - s->cs_base);
         } else {
             modrm = cpu_ldub_code(cpu_single_env, s->pc++);
-            if ((modrm & 0xc0) != 0xc0)
-                goto illegal_op;
+            /* Ignore the mod bits (assume (modrm&0xc0)==0xc0).
+             * AMD documentation (24594.pdf) and testing of
+             * intel 386 and 486 processors all show that the mod bits
+             * are assumed to be 1's, regardless of actual values.
+             */
             rm = (modrm & 7) | REX_B(s);
             reg = ((modrm >> 3) & 7) | rex_r;
             if (CODE64(s))
@@ -7594,8 +7597,11 @@ static target_ulong disas_insn(DisasContext *s, target_ulong pc_start)
             gen_exception(s, EXCP0D_GPF, pc_start - s->cs_base);
         } else {
             modrm = cpu_ldub_code(cpu_single_env, s->pc++);
-            if ((modrm & 0xc0) != 0xc0)
-                goto illegal_op;
+            /* Ignore the mod bits (assume (modrm&0xc0)==0xc0).
+             * AMD documentation (24594.pdf) and testing of
+             * intel 386 and 486 processors all show that the mod bits
+             * are assumed to be 1's, regardless of actual values.
+             */
             rm = (modrm & 7) | REX_B(s);
             reg = ((modrm >> 3) & 7) | rex_r;
             if (CODE64(s))
