errors on platforms without unaligned memory access support

[markwkidd]

The Readme file provided by the developers who first ported MAME 0.78 to libretro refers to "errors on platforms without unaligned memory access support."

That Readme file also included a link to diff file which records efforts taken to address the unaligned memory issue in xmame 0.106's source code: https://code.oregonstate.edu/svn/dsp_bd/uclinux-dist/trunk/user/games/xmame/xmame-0.106/src/unix/contrib/patches/word-align-patch

Now that this source code is managed with a github repository, I'm moving this Issue from the Readme to a place where it is more likely to be discussed and explored in the future.

[markwkidd]

The Oregon State code linked above is hosted on a site with an SSL certificate that expired in 2013. For safekeeping I'm going to paste the diff below as it is not terribly long.

diff -ur mame-0.35.orig.fixed/src/memory.c xmame-0.35.4/src/memory.c
--- mame-0.35.orig.fixed/src/memory.c   Sat Jul 10 13:49:25 1999
+++ xmame-0.35.4/src/memory.c   Sat Jul 10 14:33:20 1999
@@ -130,6 +130,58 @@

 ***************************************************************************/

+#ifdef ALIGN_SHORTS
+/*
+ * Previously READ_WORD and WRITE_WORDS were implemented as macros.
+ * However they assumed that unaligned loads are legal, which is not
+ * true on most non-x86 hardware.  Even on a Pentium, unaligned loads are
+ * slower than aligned loads.
+ */
+
+#ifdef LSB_FIRST
+#ifndef __GNUC__
+/* don't include it for gcc, gcc has a way to tell the compiler
+   to generate a unaligned access sequence (see memory.h) */
+
+int
+READ_WORD(void *dst)
+{
+  unsigned char *p = (unsigned char *) dst;
+  return (p[1] << 8) | p[0];
+}
+
+int
+WRITE_WORD(void *dst, int val)
+
+{
+  unsigned char *p = (unsigned char *) dst;
+  p[0] = val & 0xff;
+  p[1] = (val >> 8) & 0xff;
+  return val;
+}
+
+#endif /* #ifndef __GNUC__ */
+#else
+
+int
+READ_WORD(void *dst)
+{
+  unsigned char *p = (unsigned char *) dst;
+  return (p[0] << 8) | p[1];
+}
+
+int
+WRITE_WORD(void *dst, int val)
+
+{
+  unsigned char *p = (unsigned char *) dst;
+  p[1] = val & 0xff;
+  p[0] = (val >> 8) & 0xff;
+  return val;
+}
+#endif
+#endif
+
 int mrh_ram(int address){return RAM[address];}
 int mrh_bank1(int address){return cpu_bankbase[1][address];}
 int mrh_bank2(int address){return cpu_bankbase[2][address];}
diff -ur mame-0.35.orig.fixed/src/memory.h xmame-0.35.4/src/memory.h
--- mame-0.35.orig.fixed/src/memory.h   Sat Jul 10 13:49:46 1999
+++ xmame-0.35.4/src/memory.h   Sat Jul 10 14:33:20 1999
@@ -211,8 +211,25 @@

 /* ----- 16-bit memory access macros ----- */

+#ifdef ALIGN_SHORTS
+#if defined(LSB_FIRST) && defined(__GNUC__)
+
+/* special way for gcc to tell it to create unaliged access sequence */
+typedef struct {
+  unsigned short __x __attribute__((packed));
+} packed_word;
+#define READ_WORD(a)    (((packed_word *)(a))->__x)
+#define WRITE_WORD(a,d) ((((packed_word *)(a))->__x) = (d))
+
+#else
+/* Use these to avoid alignment problems on non-x86 hardware. */
+extern int READ_WORD(void *dst);
+extern int WRITE_WORD(void *dst, int d);
+#endif
+#else
 #define READ_WORD(a)          (*(unsigned short *)(a))
 #define WRITE_WORD(a,d)       (*(unsigned short *)(a) = (d))
+#endif

 #define COMBINE_WORD(w,d)     (((w) & ((d) >> 16)) | ((d) & 0xffff))
 #define COMBINE_WORD_MEM(a,d) (WRITE_WORD ((a), (READ_WORD (a) & ((d) >> 16)) | (d)))

[markwkidd]

@grant2258 do you think there is any value in studying the unaligned memory access diff I have pasted above?