Freescale S08

The 9S08 (68HCS08 or S08 for short) is a 8-bit microcontroller (µC) family originally from Motorola, now produced by Freescale Semiconductor, descended from the Motorola 6800 microprocessor. It is a CISC microcontroller. The 9S08 devices are more powerful and usually less expensive than the aging 68HC05 microcontrollers, and are found in almost any type of embedded systems. The larger members, which include an MMU, offer up to 128KB of paged flash, and 8KB of RAM. The paging scheme used allows for a theoretical maximum of 4MB of flash.

MMU equipped variants offer two extra CPU instructions, CALL and RTC, which are used instead of JSR and RTS respectively when dealing with subroutines placed in paged memory, allowing direct page-to-page subroutine calls. In a single atomic operation, CALL saves and RTC restores not only the PC but also one extra address byte, the PPAGE (program page) byte. Because of this extra byte, and to also keep the stack balanced, a subroutine ending with RTC must always be called with CALL, even if it resides in the same memory page.

Internally, the 9S08 instruction set is upward compatible with the 6805, with the addition of stack indexed addressing modes. (Instructions using the SP register have opcodes prefixed with the byte 0x9E). It has a single eight-bit accumulator, A, one eight-bit index register, X, or one sixteen-bit index registers, HX (formed from the combination of H and X registers), a condition code register, a 16-bit stack pointer, and a program counter. For compatibility with the 6805 which does not have an H register, the most significant byte of the HX register, H, is cleared during reset, and H is the only register not stacked automatically when entering any ISR (Interrupt Service Routine). Some instructions treat the H and X registers as a combined 16-bit HX register. Unlike the 6805, the stack can be placed anywhere in memory using appropriate instructions.

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