Motorola 68000 Processor

     The TI-89 calculator uses the Motorola 68000 (68k) processor. This processor contains sixteen 32-bit registers. The first eight are Data registers that hold values associated with any sort of assembly program and they are labeled D0 to D7. The next seven are the Address registers, which can be used as software stack pointers, index registers or base address registers. The eighth address register (A7) is also known as the User Stack Pointer (USP), and this holds the top value of the stack.

     The processor contains a 32-bit Program Counter (PC), and an 8-bit Condition Code Register (CCR, similar to the "flags" register or PSW in other processors). Of the eight bits in the CCR, five are used most often: the negative bit, the zero bit, the overflow bit, the carry bit, and the extend bit. The extend bit is used for multiple-word integer operations that may require a number to be carried from word to word (unlike Intel, for example, that uses the carry bit for this process).

Here is a diagram of the various registers inside the 68k processor:

     The Motorola 68k processor is a diverse processor that can handle many different types of instructions including, but not limited to, data movement, integer and floating point arithmetic, logical operations, shift and rotate operations, and bit manipulation. To see examples of some of the instructions used by the Motorola processor, visit our "Assembly" page.

(For full documentation on the Motorola 68k processor, the Motorola 68k Family Programmer's Reference Manual is an excellent resouce. Adobe Acrobat is required to view this type of file.)