diff --git a/sys/amd64/ia32/ia32_reg.c b/sys/amd64/ia32/ia32_reg.c
index 4a068b7f61e..10c91f9c0b9 100644
--- a/sys/amd64/ia32/ia32_reg.c
+++ b/sys/amd64/ia32/ia32_reg.c
@@ -146,7 +146,9 @@ fill_fpregs32(struct thread *td, struct fpreg32 *regs)
 	struct save87 *sv_87;
 	struct env87 *penv_87;
 	struct envxmm *penv_xmm;
-	int i;
+	int i, st;
+	uint16_t tw;
+	uint8_t ab_tw;
 
 	bzero(regs, sizeof(*regs));
 	sv_87 = (struct save87 *)regs;
@@ -173,12 +175,30 @@ fill_fpregs32(struct thread *td, struct fpreg32 *regs)
 	penv_87->en_fos = td->td_frame->tf_ds;
 
 	/* FPU registers and tags */
-	penv_87->en_tw = 0xffff;
-	for (i = 0; i < 8; ++i) {
-		sv_87->sv_ac[i] = sv_fpu->sv_fp[i].fp_acc;
-		if ((penv_xmm->en_tw & (1 << i)) != 0)
-			penv_87->en_tw &= ~(3 << i * 2);
+	/* For ST(i), i = fpu_reg - top, we start with fpu_reg=7. */
+	st = 7 - ((penv_xmm->en_sw >> 11) & 7);
+	ab_tw = penv_xmm->en_tw;
+	tw = 0;
+	for (i = 0x80; i != 0; i >>= 1) {
+		sv_87->sv_ac[st] = sv_fpu->sv_fp[st].fp_acc;
+		tw <<= 2;
+		if (ab_tw & i) {
+			/* Non-empty - we need to check ST(i) */
+			struct fpacc87* fx_reg = &sv_fpu->sv_fp[st].fp_acc;
+			uint16_t exp = *((uint16_t*)&fx_reg->fp_bytes[8]) & 0x7fff;
+			uint64_t mantissa = *((uint64_t*)fx_reg->fp_bytes);
+			if (exp == 0) {
+				if (mantissa == 0)
+					tw |= 1; /* Zero */
+				else
+					tw |= 2; /* Denormal */
+			} else if (exp == 0x7fff)
+				tw |= 2; /* Infinity or NaN */
+		} else
+			tw |= 3; /* Empty */
+		st = (st - 1) & 7;
 	}
+	penv_87->en_tw = tw;
 
 	return (0);
 }
diff --git a/sys/i386/i386/npx.c b/sys/i386/i386/npx.c
index d5ce4fe5513..285b2bf58d1 100644
--- a/sys/i386/i386/npx.c
+++ b/sys/i386/i386/npx.c
@@ -1149,7 +1149,9 @@ npx_fill_fpregs_xmm1(struct savexmm *sv_xmm, struct save87 *sv_87)
 {
 	struct env87 *penv_87;
 	struct envxmm *penv_xmm;
-	int i;
+	int i, st;
+	uint8_t ab_tw;
+	uint16_t tw;
 
 	penv_87 = &sv_87->sv_env;
 	penv_xmm = &sv_xmm->sv_env;
@@ -1164,13 +1166,31 @@ npx_fill_fpregs_xmm1(struct savexmm *sv_xmm, struct save87 *sv_87)
 	penv_87->en_fos = penv_xmm->en_fos;
 
 	/* FPU registers and tags */
-	penv_87->en_tw = 0xffff;
-	for (i = 0; i < 8; ++i) {
-		sv_87->sv_ac[i] = sv_xmm->sv_fp[i].fp_acc;
-		if ((penv_xmm->en_tw & (1 << i)) != 0)
-			/* zero and special are set as valid */
-			penv_87->en_tw &= ~(3 << i * 2);
+	/* For ST(i), i = fpu_reg - top, we start with fpu_reg=7. */
+	st = 7 - ((penv_xmm->en_sw >> 11) & 7);
+	ab_tw = penv_xmm->en_tw;
+	tw = 0;
+	for (i = 0x80; i != 0; i >>= 1) {
+		sv_87->sv_ac[st] = sv_xmm->sv_fp[st].fp_acc;
+		tw <<= 2;
+		if (ab_tw & i) {
+			/* Non-empty - we need to check ST(i) */
+			struct fpacc87* fx_reg = &sv_xmm->sv_fp[st].fp_acc;
+			uint16_t exp = *((uint16_t*)fx_reg) & 0x7fff;
+			uint16_t exp = *((uint16_t*)&fx_reg->fp_bytes[8]) & 0x7fff;
+			uint64_t mantissa = *((uint64_t*)fx_reg->fp_bytes);
+			if (exp == 0) {
+				if (mantissa == 0)
+					tw |= 1; /* Zero */
+				else
+					tw |= 2; /* Denormal */
+			} else if (exp == 0x7fff)
+				tw |= 2; /* Infinity or NaN */
+		} else
+			tw |= 3; /* Empty */
+		st = (st - 1) & 7;
 	}
+	penv_87->en_tw = tw;
 }
 
 void