An apparatus and method for providing wait states using address bits not used in the device address decode. The upper address bits of a computer system are not used for peripheral and memory device decoding purposes. The unused bits are driven to indicate the desired number of wait states to be developed for each selected device, while still allowing a normal decode of the devices. Wait state and ready logic is provided which allows each device address to be assigned one of several possible wait state lengths by driving the most significant bits of the address. The address decode based wait state determination is overridden for RAM operations, and followed for ROM and peripheral operations.

A data processor (1) includes a wait state control unit (6) for generating a ready signal for instructing a bus control unit (3) to insert a wait state into a bus cycle being performed. The bus control unit performs a bus cycle with first and second states (T1 and T2). The wait state control unit includes a bus cycle judgement circuit, a register, a wait cycle selector/decoder, a wait cycle presence/absence detector for detecting whether or not a wait state is to be inserted into the bus cycle, and a ready signal generator for generating a ready signal in response to the detection signal from the wait cycle presence/absence detector for generating the ready signal in response to the detection signal irrespective of the absence of the decoded wait cycle number from the wait cycle selector/decoder. Thus, the wait cycle presence/absence detector, which detects only the requirement of the insertion of a wait state, causes the ready signal to be generated to the bus control unit even though the number of wait states has not been decoded by the end of the second state.

A clock modulation circuit modulates the frequency of a clock signal to generate a modulated clock signal. A wait requesting signal receives frequency information indicating the frequency of the modulated clock signal and, when the frequency information indicates a frequency higher than a reference frequency, generates a wait requesting signal to an external bus interface. Since an optimum wait cycle is inserted to the external bus interface according to a change of the frequency of the modulated clock signal, needless wait cycle can be prevented from being inserted to the external bus interface. As a result of this, it is possible to disperse the peak of radiated noise which is caused by the clock signal and to reduce electromagnetic interference, without decreasing performance of a system. Namely, it can serve both market needs for reducing noise and speeding up.

A microprocessor according to the present invention comprises a sub-read bus, to which output terminals of registers of a register file of the microprocessor are coupled. The sub-read bus is in turn coupled to a main read bus of the microprocessor through a bus output circuit. Upon occurrence of a read access to any of the registers, the bus output circuit couples the sub-read bus with the main read bus, whereby data read out from the registers to the sub-read bus are transmitted to the main read bus, and under no existence of the read access, the bus output circuit interrupts the data transmission from the sub-read bus to the main read bus. With this, a load capacitance of the read bus is reduced. As a result, a time for making access to the read bus is much improved.

An advanced configuration and power interface operating system transparent method to control the wake-to-sleep and sleep-to-wake transitions includes: detecting a sleep enable command; temporarily blocking completion of the sleep enable command; generating an interrupt; configuring an input-output device; and completing the sleep enable command. The sleep enable command can be a write command to an advanced configuration and power interface sleep enable data storage unit. The generated interrupt can be a system management interrupt that invokes a basic input-output device configuration program.