Special 486 build

cf: https://x.com/RetroTechChris/status/1927145070752403698

build/i586/Makefile

@@ -10,7 +10,7 @@ else
   GIT_AVAILABLE = true
 endif
 
-CFLAGS = -std=gnu11 -Wall -Wextra -Wshadow -m32 -march=i586 -fpic -fno-builtin \
+CFLAGS = -std=gnu11 -Wall -Wextra -Wshadow -m32 -march=i486 -fpic -fno-builtin \
          -ffreestanding -fomit-frame-pointer -fno-stack-protector \
          -fexcess-precision=standard -DARCH_BITS=32
 

lib/unistd.c

@@ -36,7 +36,7 @@ void usleep(unsigned int usec)
         } while ((get_tsc() - t0) < cycles);
     } else {
         // This will be highly inaccurate, but should give at least the requested delay.
-        volatile uint64_t count = (uint64_t)usec * 1000;
+        volatile uint64_t count = (uint64_t)usec * 10;
         while (count > 0) {
             count--;
         }

system/x86/cpuinfo.c

@@ -97,6 +97,13 @@ static void determine_cache_size()
           break;
         }
         /* fall through */
+      case 'U':
+        // UMC UMC UMC
+        if (cpuid_info.vendor_id.str[0] == 'U') {
+          l1_cache = 8;
+          break;
+        }
+        /* fall through */
       case 'G':
         if (cpuid_info.vendor_id.str[9] == 'N') {
           // National Semiconductor
@@ -899,7 +906,17 @@ static void determine_cpu_model(void)
             }
         }
         break;
-
+      case 'U':
+        // UMC UMC UMC
+        switch (cpuid_info.version.family) {
+          case 4:
+            l1_cache = 8;
+            cpu_model = "UMC Green CPU U5S 486";
+            break;
+          default:
+            break;
+        }
+        break;
       case 'V':
         // Vortex86 SoC
         switch (cpuid_info.version.family) {
@@ -933,6 +950,9 @@ static void determine_cpu_model(void)
       default:
         // Unknown processor - make a guess at the family.
         switch (cpuid_info.version.family) {
+          case 4:
+            cpu_model = "486-class CPU (unknown)";
+            break;
           case 5:
             cpu_model = "586-class CPU (unknown)";
             break;

tests/tests.c

@@ -64,7 +64,7 @@ test_pattern_t test_list[NUM_TEST_PATTERNS] = {
     { true,  PAR,    1,   81,    0, "[Block move]                           "},
     { true,  PAR,    1,   48,    0, "[Random number sequence]               "},
     { true,  PAR,    1,    6,    0, "[Modulo 20, random pattern]            "},
-    { true,  ONE,    6,  240,    0, "[Bit fade test, 2 patterns]            "},
+    { false, ONE,    6,  240,    0, "[Bit fade test, 2 patterns]            "},
 };
 
 int ticks_per_pass[NUM_PASS_TYPES];