diff --git a/scsp.h b/scsp.h
index 1ed76ea..2d84599 100644
--- a/scsp.h
+++ b/scsp.h
@@ -14,13 +14,84 @@ static_assert((sizeof (union scsp_ram)) == 0x080000);
 
 typedef unsigned char scsp_res0[0x080000];
 
+// Formal register names are poorly documented; these are as faithfully as
+// possible derived from "4.2 Sound Source Register"
 typedef struct scsp_slot {
-  reg16 field[0x18 / 2];
+  struct {     // Loop control
+    reg16 LOOP;
+    reg16 SA;  // start address
+    reg16 LSA; // loop start address
+    reg16 LEA; // loop end address
+  };
+  union {      // EG register
+    struct {
+      reg16 EGU;
+      reg16 EGL;
+    };
+    reg32 EG;
+  };
+  reg16 VOLUME; // Sound volume
+  reg16 FM;     // FM modulation control
+  reg16 PITCH;  // FM pitch control
+  reg16 LFO;    // LFO (low-freqency oscillator)
+  union {       // slot mixer
+    struct {
+      reg16 MIXERU;
+      reg16 MIXERL;
+    };
+    reg32 MIXER;
+  };
   reg16 _res[0x8 / 2];
 } scsp_slot;
 
 static_assert((sizeof (struct scsp_slot)) == 0x20);
 
+// Formal register names are poorly documented; these are as faithfully as
+// possible derived from "4.2 Sound Source Register"
+typedef struct scsp_ctrl {
+  reg16 MIXER;     // master mixer
+  reg16 DSP;       // DSP memory control
+  union {          // MIDI
+    struct {
+      reg16 MIDIU;
+      reg16 MIDIL;
+    };
+    reg32 MIDI;
+  };
+  reg16 STATUS;    // slot status
+  reg16 _res[(8 / 2)];
+  struct {
+    reg16 DMEA;
+    reg16 DMAU;
+    reg16 DMAL;
+  };
+  struct {
+    reg16 TIMA;
+    reg16 TIMB;
+    reg16 TIMC;
+  };
+  struct {
+    reg16 SCIEB;
+    reg16 SCIPD;
+    reg16 SCIRE;
+  };
+  struct {
+    reg16 SCILV0;
+    reg16 SCILV1;
+    reg16 SCILV2;
+  };
+  struct {
+    reg16 MCIEB;
+    reg16 MCIPD;
+    reg16 MCIRE;
+  };
+} scsp_ctrl;
+
+static_assert((sizeof (struct scsp_ctrl)) == 0x30);
+static_assert((offsetof (struct scsp_ctrl, STATUS)) == 0x08);
+static_assert((offsetof (struct scsp_ctrl, DMEA)) == 0x12);
+static_assert((offsetof (struct scsp_ctrl, MCIRE)) == 0x2e);
+
 typedef struct scsp_dsp {
   reg16 COEF[64];
   reg16 MADRS[32];
@@ -33,18 +104,18 @@ static_assert((sizeof (struct scsp_dsp)) == 0x7e4);
 
 typedef struct scsp_reg {
   scsp_slot slot[32];
-  reg16 common[0x30 / 2];
-  reg16 _res0[0x1d0 / 2];
+  scsp_ctrl ctrl;
+  reg16 _res0[(0x1d0 / 2)];
   struct {
     reg16 gen_a[32];
     reg16 gen_b[32];
   } direct;
-  reg16 _res1[0x80 / 2];
+  reg16 _res1[(0x80 / 2)];
   scsp_dsp dsp;
 } scsp_reg;
 
 static_assert((sizeof (struct scsp_reg)) == 0x000ee4);
-static_assert((offsetof (struct scsp_reg, common)) == 0x000400);
+static_assert((offsetof (struct scsp_reg, ctrl)) == 0x000400);
 static_assert((offsetof (struct scsp_reg, direct)) == 0x000600);
 static_assert((offsetof (struct scsp_reg, dsp)) == 0x000700);
 
@@ -62,7 +133,64 @@ static_assert((offsetof (struct scsp, reg)) == 0x100000);
 
 // bits
 
-enum scsp_bits {
-  SCSP__0__MEM4MB = (1 << 9),
-  SCSP__0__DAC18B = (1 << 8),
+enum slot_bits {
+  LOOP__KYONEX = (    1 << 12), // (KX) execute KEY_ON
+  LOOP__KYONB = (     1 << 11), // (KB) record KEY_ON, KEY_OFF
+  LOOP__SBCTL = (  0b00 << 10), // source bit control
+  LOOP__SSCTL = (  0b00 << 8 ), // sound source control
+  LOOP__LPCTL = (  0b00 << 6 ), // loop control
+  LOOP__PCM8B = (     1 << 5 ), // (8B) 8bit signed PCM
+  LOOP__SA =    (0b0000 << 0 ), // start address
+};
+
+enum eg_bits {
+  EG__D2R    = (0b00000 << 27), // Decay 2 rate
+  EG__D1R    = (0b00000 << 22), // Decay 1 rate
+  EG__EGHOLD = (      1 << 21), // (HO) EG hold mode
+  EG__AR     = (0b00000 << 16), // Attack rate
+  EG__LPSLNK = (      1 << 14), // (LS) Loop start link
+  EG__KRS    = ( 0b0000 << 10), // Key rate scaling
+  EG__DL     = (0b00000 << 5 ), // Decay level
+  EG__RR     = (0b00000 << 0 ), // Release rate
+};
+
+enum volume_bits {
+  VOLUME__STWINH = (1 << 9), // (SI) Stack write inhibit (FM-related register)
+  VOLUME__SDIR   = (1 << 8), // (SD) Sound direct
+#define FM__TL(n) ((n) << 0) // Total level
+};
+
+enum fm_bits {
+  FM__MDL   = (  0b0000 << 12), // Modulation level
+  FM__MDXSL = (0b000000 << 6 ), // Select modulation input X
+  FM__MDYSL = (0b000000 << 0 ), // Select modulation input Y
+};
+
+enum lfo_bits {
+  LFO__LFORE  = (      1 << 15), // (RE) LFO reset
+  LFO__LFOF   = (0b00000 << 10), // LFO frequency
+  LFO__PLFOWS = (   0b00 << 8 ), // Pitch-LFO wave select
+  LFO__PLFOS  = (  0b000 << 5 ), // Pitch-LFO sensitivity
+  LFO__ALFOWS = (   0b00 << 3 ), // Amplitude-LFO wave select
+  LFO__ALFOS  = (  0b000 << 0 ), // Amplitude-LFO sensitivity
+};
+
+enum mixer_bits {
+  MIXER__ISEL  = ( 0b0000 << 19),
+  MIXER__IMXL  = (  0b000 << 16),
+  MIXER__DISDL = (  0b000 << 13),
+  MIXER__DIPAN = (0b00000 << 8 ),
+  MIXER__EFSDL = (  0b000 << 5 ),
+  MIXER__EFPAN = (0b00000 << 0 ),
+};
+
+//enum pitch_bits {
+#define PITCH__OCT(n) ((n) << 11)
+#define PITCH__FNS(n) ((n) << 0 )
+//};
+
+enum scsp_bits {
+  MIXER__MEM4MB = (1 << 9),
+  MIXER__DAC18B = (1 << 8),
+#define MIXER__MVOL(n) ((n) << 0)
 };