Fixed RISC division algorithm; thanks to SPCPD for the info.

[virtualjaguar] / src / gpu.cpp

diff --git a/src/gpu.cpp b/src/gpu.cpp
index d5e9054c13716717ce47cce6b86be120920fb268..fbf26a0e6846fba758246ee0cec664eaf677f044 100644 (file)
--- a/src/gpu.cpp
+++ b/src/gpu.cpp
@@ -2390,13 +2390,7 @@ static void gpu_opcode_div(void) // RN / RM
        if (doGPUDis)
                WriteLog("%06X: DIV    R%02u, R%02u (%s) [NCZ:%u%u%u, R%02u=%08X, R%02u=%08X] -> ", gpu_pc-2, IMM_1, IMM_2, (gpu_div_control & 0x01 ? "16.16" : "32"), gpu_flag_n, gpu_flag_c, gpu_flag_z, IMM_1, RM, IMM_2, RN);
 #endif
-// NOTE: remainder is NOT calculated correctly here!
-//       The original tried to get it right by checking to see if the
-//       remainder was negative, but that's too late...
-// The code there should do it now, but I'm not 100% sure...
-// [Now it should be correct, but not displaying correct behavior of the actual
-//  hardware. A step in the right direction.]
-
+#if 0
        if (RM)
        {
                if (gpu_div_control & 0x01)             // 16.16 division
@@ -2411,16 +2405,35 @@ static void gpu_opcode_div(void)        // RN / RM
                        gpu_remain = RN % RM;
                        RN = RN / RM;
                }
-
-// What we really should do here is figure out why this condition
-// happens in the real divide unit and emulate *that* behavior.
-#if 0
-               if ((gpu_remain - RM) & 0x80000000)     // If the result would have been negative...
-                       gpu_remain -= RM;                       // Then make it negative!
-#endif
        }
        else
+       {
+               // This is what happens according to SCPCD. NYAN!
                RN = 0xFFFFFFFF;
+               gpu_remain = 0;
+       }
+#else
+       // Real algorithm, courtesy of SCPCD: NYAN!
+       uint32_t q = RN;
+       uint32_t r = 0;
+
+       // If 16.16 division, stuff top 16 bits of RN into remainder and put the
+       // bottom 16 of RN in top 16 of quotient
+       if (gpu_div_control & 0x01)
+               q <<= 16, r = RN >> 16;
+
+       for(int i=0; i<32; i++)
+       {
+//             uint32_t sign = (r >> 31) & 0x01;
+               uint32_t sign = r & 0x80000000;
+               r = (r << 1) | ((q >> 31) & 0x01);
+               r += (sign ? RM : -RM);
+               q = (q << 1) | (((~r) >> 31) & 0x01);
+       }
+
+       RN = q;
+       gpu_remain = r;
+#endif
 
 #ifdef GPU_DIS_DIV
        if (doGPUDis)