mac_via: fix rtc command decoding for the PRAM seconds registers

Analysis of the MacOS toolbox ROM code shows that on startup it attempts 2
separate reads of the seconds registers with commands 0x9d...0x91 followed by
0x8d..0x81 without resetting the command to its initial value. The PRAM seconds
value is only accepted when the values of the 2 separate reads match.

From this we conclude that bit 4 of the rtc command is not decoded or we don't
care about its value when reading the PRAM seconds registers. Implement this
decoding change so that both reads return successfully which allows the MacOS
toolbox ROM to correctly set the date/time.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20230621085353.113233-25-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/misc/mac_via.c

@@ -362,10 +362,10 @@ static void pram_update(MOS6522Q800VIA1State *v1s)
  *
  * Command byte    Register addressed by the command
  *
- * z0000001        Seconds register 0 (lowest-order byte)
- * z0000101        Seconds register 1
- * z0001001        Seconds register 2
- * z0001101        Seconds register 3 (highest-order byte)
+ * z00x0001        Seconds register 0 (lowest-order byte)
+ * z00x0101        Seconds register 1
+ * z00x1001        Seconds register 2
+ * z00x1101        Seconds register 3 (highest-order byte)
  * 00110001        Test register (write-only)
  * 00110101        Write-Protect Register (write-only)
  * z010aa01        RAM address 100aa ($10-$13) (first 20 bytes only)
@@ -373,6 +373,7 @@ static void pram_update(MOS6522Q800VIA1State *v1s)
  * z0111aaa        Extended memory designator and sector number
  *
  * For a read request, z=1, for a write z=0
+ * The letter x indicates don't care
  * The letter a indicates bits whose value depend on what parameter
  * RAM byte you want to address
  */
@@ -389,7 +390,7 @@ static int via1_rtc_compact_cmd(uint8_t value)
     }
     if ((value & 0x03) == 0x01) {
         value >>= 2;
-        if ((value & 0x1c) == 0) {
+        if ((value & 0x18) == 0) {
             /* seconds registers */
             return read | (REG_0 + (value & 0x03));
         } else if ((value == 0x0c) && !read) {