From 3358d957043c726957ea6104d34f7b166195db5e Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Fri, 19 May 2023 14:52:15 -0700
Subject: [PATCH] scsp: zeroize dsp

The Saturn BIOS does not (un)initialize the DSP. Without zeroizing the DSP
program, the SCSP DSP appears to have a program that continuously writes to
0x30000 through 0x3ffff in sound RAM, which has the effect of destroying any
samples stored there.
---
 common/copy.hpp               |  9 +++++++++
 scsp/sound_cpu__interrupt.cpp | 11 +++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/common/copy.hpp b/common/copy.hpp
index 4df90fc..d9c1274 100644
--- a/common/copy.hpp
+++ b/common/copy.hpp
@@ -8,3 +8,12 @@ inline void copy(T * dst, const T * src, int32_t n) noexcept
     n -= (sizeof (T));
   }
 }
+
+template <typename T>
+inline void fill(T * dst, const T src, int32_t n) noexcept
+{
+  while (n > 0) {
+    *dst++ = src;
+    n -= (sizeof (T));
+  }
+}
diff --git a/scsp/sound_cpu__interrupt.cpp b/scsp/sound_cpu__interrupt.cpp
index 1cdc707..bccae66 100644
--- a/scsp/sound_cpu__interrupt.cpp
+++ b/scsp/sound_cpu__interrupt.cpp
@@ -15,23 +15,26 @@ void main()
      The document suggests that Sound RAM is (somewhat) preserved
      during SNDOFF.
    */
-  
+
   while ((smpc.reg.SF & 1) != 0);
   smpc.reg.SF = 1;
   smpc.reg.COMREG = COMREG__SNDOFF;
   while (smpc.reg.oreg[31] != OREG31__SNDOFF);
 
   scsp.reg.ctrl.MIXER = MIXER__MEM4MB;
-  
+
+  reg32 * dsp_steps = reinterpret_cast<reg32*>(&(scsp.reg.dsp.STEP[0].MPRO[0]));
+  fill<reg32>(dsp_steps, 0, (sizeof (scsp.reg.dsp.STEP)));
+
   uint32_t * m68k_main_start = reinterpret_cast<uint32_t*>(&_m68k_start);
   uint32_t m68k_main_size = reinterpret_cast<uint32_t>(&_m68k_size);
   copy<uint32_t>(&scsp.ram.u32[0], m68k_main_start, m68k_main_size);
-  
+
   while ((smpc.reg.SF & 1) != 0);
   smpc.reg.SF = 1;
   smpc.reg.COMREG = COMREG__SNDON;
   while (smpc.reg.oreg[31] != OREG31__SNDON);
-  
+
   // do nothing while the sound CPU manipulates the SCSP
 }