Abstract: An extensible central processing unit (CPU) (12 or 12'). By modifying the architecture of a new or prior art CPU, a new or prior art CPU can be made extensible so that new instructions can be added in a simple and cost effective manner to meet differing customer needs. The term "extensible" in regard to a CPU is used to mean that new instructions can be added to the CPU simply by adding certain designated circuitry, without the need to significantly change the existing CPU circuitry. In some embodiments, the additional designated circuitry may include control circuitry in the form of CPU control extension circuitry (52 or 152). In some embodiments, the additional circuitry may include non-control circuitry in the form of execution unit extension circuitry (153).

Abstract: A data processing system providing an extensible register and method thereof. A new CPU has an extensible index register. The new CPU is object code compatible with the old CPU having an 8-bit index register, yet the index register of the new CPU can be effectively extended to 16 bits when new instructions are used. As a consequence, the user is able to make the choice between using assembly code software written for the old CPU and having the functionality of an 8-bit index register, or writing new assembly code software for the new CPU and having the functionality of a 16-bit index register.

Abstract: A data processor (10) increments a sixteen bit program counter value using an arithmetic logic unit, ALU, (224) and an eight bit incrementer(250). The ALU increments a low byte of the program counter value. A carry generated by incrementing the low byte is propagated to the incrementer. The incrementer then increments the high byte of the program counter value. Subsequently, the high and low bytes of the program counter value are respectively stored in a high and low program counter register (200, 206). Therefore, eight bits of an incrementer which would have typically been required to implement an incrementer for the low byte of the program counter value have been eliminated without a reduction in functionality of the data processor.

Abstract: A data processing system (10) executes a branch instruction in a straight line microcode sequence. During execution of the instruction, a control unit (56) is provided to decode the instruction to provide a plurality of control signals and to determine a data value with which to test a condition of the instruction. A constants generation circuit (222) generates a mask value in response to the plurality of control signals. An arithmetic logic unit (224) logically combines the mask value with the data value to produce a masked value. A zero detect circuit (246) subsequently tests the masked value to determine if it is equal to zero and provides a condition signal to indicate if the condition of the instruction was satisfied. If the condition of the instruction is satisfied, an offset value plus one is added to a program counter value. Otherwise, the program counter value is incremented by one.