diff --git a/sys/amd64/amd64/identcpu.c b/sys/amd64/amd64/identcpu.c
index a7142d4126c..63a05605456 100644
--- a/sys/amd64/amd64/identcpu.c
+++ b/sys/amd64/amd64/identcpu.c
@@ -476,6 +476,7 @@ printcpuinfo(void)
 		switch (cpu_id & 0xff0) {
 		case 0x540:
 			strcat(cpu_model, "WinChip C6");
+			tsc_is_broken = 1;
 			break;
 		case 0x580:
 			strcat(cpu_model, "WinChip 2");
diff --git a/sys/i386/i386/identcpu.c b/sys/i386/i386/identcpu.c
index a7142d4126c..63a05605456 100644
--- a/sys/i386/i386/identcpu.c
+++ b/sys/i386/i386/identcpu.c
@@ -476,6 +476,7 @@ printcpuinfo(void)
 		switch (cpu_id & 0xff0) {
 		case 0x540:
 			strcat(cpu_model, "WinChip C6");
+			tsc_is_broken = 1;
 			break;
 		case 0x580:
 			strcat(cpu_model, "WinChip 2");