diff --git a/src/runtime/memclr_386.s b/src/runtime/memclr_386.s
index 8b163923e9..1520aea2e0 100644
--- a/src/runtime/memclr_386.s
+++ b/src/runtime/memclr_386.s
@@ -6,6 +6,8 @@
 
 #include "textflag.h"
 
+// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
+
 // void runtime·memclr(void*, uintptr)
 TEXT runtime·memclr(SB), NOSPLIT, $0-8
 	MOVL	ptr+0(FP), DI
diff --git a/src/runtime/memclr_amd64.s b/src/runtime/memclr_amd64.s
index 35b3b4beef..94a2c7f236 100644
--- a/src/runtime/memclr_amd64.s
+++ b/src/runtime/memclr_amd64.s
@@ -6,6 +6,8 @@
 
 #include "textflag.h"
 
+// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
+
 // void runtime·memclr(void*, uintptr)
 TEXT runtime·memclr(SB), NOSPLIT, $0-16
 	MOVQ	ptr+0(FP), DI
diff --git a/src/runtime/sys_windows_386.s b/src/runtime/sys_windows_386.s
index ebcfdf4e0d..9b1fc7a205 100644
--- a/src/runtime/sys_windows_386.s
+++ b/src/runtime/sys_windows_386.s
@@ -191,7 +191,7 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
 	LEAL	g_end(SP), BX
 	MOVL	BX, g_m(SP)
-	LEAL	-4096(SP), CX
+	LEAL	-8192(SP), CX
 	MOVL	CX, (g_stack+stack_lo)(SP)
 	ADDL	$const_StackGuard, CX
 	MOVL	CX, g_stackguard0(SP)