vcs.RunForFrameCount() checks for CPU KIL state

hardware/run.go

@@ -110,6 +110,12 @@ func (vcs *VCS) RunForFrameCount(numFrames int, continueCheck func(frame int) (g
 
 	state := govern.Running
 	for frameNum != targetFrame && state != govern.Ending {
+		// check if CPU has been killed. emulation will run forever if we don't
+		// check for this
+		if vcs.CPU.Killed {
+			return nil
+		}
+
 		err := vcs.Step(nil)
 		if err != nil {
 			return err

preview/doc.go

@@ -15,4 +15,8 @@
 
 // Package preview is used to emulate a ROM for a short period of time in order
 // to gather information that would takes time to acquire.
+//
+// For example, it is used to determine the initial framing for the television
+// on startup. Without a preview emulation the default framing will most often
+// be wrong, resulting in a visible change in the image.
 package preview

preview/preview.go

@@ -28,7 +28,7 @@ type Emulation struct {
 	vcs *hardware.VCS
 }
 
-// NewPreview is the preferred method of initialisation for the Preview type
+// NewEmulation is the preferred method of initialisation for the Emulation type
 func NewEmulation(prefs *preferences.Preferences) (*Emulation, error) {
 	em := &Emulation{}