This chapter introduces the std_logic_1164 logic system used for all gate-level and system-level simulation components in Hades. Where the traditional Boolean logic only supports the binary values (0/1), the std_logic system includes a total of nine logic values. The extra values are used to model undefined, illegal, or weakly driven signals and cover most situations encountered in digital logic design.

Most circuits only use the values 0 and 1 during a simulation, while U indicates signals that have not yet been initialized, and X indicates real malfunctions. The remaining values, namely Z, W, L, and H, are necessary to model signals (buses) with multiple drivers. The special D value is not used at all during simulation, but provided to support logic synthesis tools.

	Logic values and glow-mode colors	applet	webstart	print
	Basic std_logic_1164 functions	applet	webstart	print
	Interactive switches	applet	webstart	print
	Basic gates	applet	webstart	print
	Hexadecimal (4 bit) switch and display	applet	webstart	print
	Clock generator component	applet	webstart	print
	Two-phase clock generator	applet	webstart	print
	Buses and the std_logic_1164 resolution function	applet	webstart	print
	Wired-AND bus demonstration	applet	webstart	print
	Detectors and counters demonstration	applet	webstart	print