There is no "direct" access to the alternate registers; instead, two special instructions, codice_15 and codice_16, each toggles one of two multiplexer flip-flops. This enables fast context switches for interrupt service routines: codice_17 may be used alone, for really simple and fast interrupt routines, or together with codice_16 to swap the whole BC, DE, HL set. This is still several times as fast as pushing the same registers on the stack. Slower, lower priority, or multi level interrupts normally use the stack to store registers, however.