diff --git a/Source/Core/DSPCore/Src/DSPInterpreter.cpp b/Source/Core/DSPCore/Src/DSPInterpreter.cpp
index 47993339be..b23e636593 100644
--- a/Source/Core/DSPCore/Src/DSPInterpreter.cpp
+++ b/Source/Core/DSPCore/Src/DSPInterpreter.cpp
@@ -510,7 +510,7 @@ void si(const UDSPInstruction& opc)
 
 // TSTAXH $axR.h
 // 1000 011r xxxx xxxx
-// Test hight part of secondary accumulator $axR.h.
+// Test high part of secondary accumulator $axR.h.
 void tstaxh(const UDSPInstruction& opc)
 {
 	u8 reg  = (opc.hex >> 8) & 0x1;
@@ -519,15 +519,17 @@ void tstaxh(const UDSPInstruction& opc)
 	Update_SR_Register16(val);
 }
 
-// TSTAXL $axR.h
-// 1000 011r xxxx xxxx
-// Test lower part of secondary accumulator $axR.h.
+// TSTAXL $acR
+// 1000 r001 xxxx xxxx
+// r specifies one of the main accumulators.
+// Definitely not a test instruction - it changes the accums.
+// Not affected by m0/m2. Not affected by s16/s40.
 void tstaxl(const UDSPInstruction& opc)
 {
-	u8 reg  = (opc.hex >> 8) & 0x1;
-	s16 val = dsp_get_ax_l(reg);
-
-	Update_SR_Register16(val);
+	// This is probably all wrong.
+	//u8 reg  = (opc.hex >> 8) & 0x1;
+	//s16 val = dsp_get_ax_l(reg);
+	//Update_SR_Register16(val);
 }
 
 // CLR $acR
diff --git a/Source/Core/DSPCore/Src/DSPTables.cpp b/Source/Core/DSPCore/Src/DSPTables.cpp
index e57c886fe2..73d78ee658 100644
--- a/Source/Core/DSPCore/Src/DSPTables.cpp
+++ b/Source/Core/DSPCore/Src/DSPTables.cpp
@@ -240,8 +240,8 @@ const DSPOPCTemplate opcodes[] =
 
 	{"TST",		0xb100, 0xf7ff, DSPInterpreter::tst,   nop, 1 | P_EXT, 1, {{P_ACC, 1, 0, 11, 0x0800}}, dsp_op_ext_ops_pro, dsp_op_ext_ops_epi},
 
-	// GUESSING NOT SURE AT ALL!!!!
-	{"TSTAXL",  0xa100, 0xffff, DSPInterpreter::tstaxl, nop, 1 | P_EXT, 1, {{P_REG1A, 1, 0, 8, 0x0100}}, dsp_op_ext_ops_pro, dsp_op_ext_ops_epi},
+	// Definitely not TSTAXL, it affects one of the accumulators. (a100 or a900, same op, one parameter).
+	{"TSTAXL",  0xa100, 0xffff, DSPInterpreter::tstaxl, nop, 1 | P_EXT, 1, {{P_ACC, 1, 0, 11, 0x0800}}, dsp_op_ext_ops_pro, dsp_op_ext_ops_epi},
 
 	{"TSTAXH",  0x8600, 0xfeff, DSPInterpreter::tstaxh, nop, 1 | P_EXT, 1, {{P_REG1A, 1, 0, 8, 0x0100}}, dsp_op_ext_ops_pro, dsp_op_ext_ops_epi},
 
diff --git a/Source/Core/DSPCore/Src/gdsp_opcodes_helper.h b/Source/Core/DSPCore/Src/gdsp_opcodes_helper.h
index ee6b02d552..36b0e9a4d5 100644
--- a/Source/Core/DSPCore/Src/gdsp_opcodes_helper.h
+++ b/Source/Core/DSPCore/Src/gdsp_opcodes_helper.h
@@ -167,7 +167,7 @@ inline void dsp_set_long_acc(int _reg, s64 val)
 	val >>= 16;
 	g_dsp.r[0x1e + _reg] = (u16)val;
 	val >>= 16;
-	g_dsp.r[0x10 + _reg] = (u16)val;
+	g_dsp.r[0x10 + _reg] = (u16)(s16)(s8)(u8)val;
 }
 
 
diff --git a/Source/DSPSpy/dsp_code.ds b/Source/DSPSpy/dsp_code.ds
index 9076cc0ebd..9808736332 100644
--- a/Source/DSPSpy/dsp_code.ds
+++ b/Source/DSPSpy/dsp_code.ds
@@ -185,16 +185,37 @@ main:
 
 	nop
 	nop
-	nop
+	lris $AC0.M, #0xcc
+	lris $AC1.M, #0xcc
 	nop
 	
-	inc $acc0
+	mrr $r00, $r13
     call send_back
 
 	s16
-	inc $acc0
+	nop
+	lris $AC0.M, #0xcc
+	lris $AC1.M, #0xcc
+	nop
+	nop
+	mrr $r00, $r13
     call send_back
 		
+	cw 0xa100
+	
+	call send_back
+
+	cw 0xa900
+	
+	call send_back
+	s16
+	cw 0xa100
+	
+	call send_back
+	s16
+	cw 0xa900
+	
+	call send_back
 ; We're done - currently we only test one opcode, in this case 0x8600.
 ; It's possible to test many more in one go - just call send_back after each one.
 
@@ -623,7 +644,8 @@ dma_copy:
 	
 	ret   ; from send_back
 	
-; I wish I knew what this 16 means.
+; If you are in s16 mode, use this instead of send_back if you want to stay
+; in s16 mode.
 send_back_16:
 	set40
 	call	send_back