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Motorola 68000

The Motorola 68000 (\\\\\\\\\\\\\\\"\\\\\\\\\\\\\\\'sixty-eight-thousand\\\\\\\\\\\\\\\'\\\\\\\\\\\\\\\"; also called the Motorola 68k, \\\\\\\\\\\\\\\"sixty-eight-k\\\\\\\\\\\\\\\") is a 16/32-bit[1] CISC microprocessor core designed and marketed by Motorola Semiconductor Products Sector (now Freescale Semiconductor). Introduced in 1979 with HMOS technology as the first member of the successful 32-bit m68k family of microprocessors, it is generally software forward compatible with the rest of the line despite being limited to a 16-bit wide external bus. After 30 years in production, the 68000 architecture is still in use.

Address bus

The 68000 has a 24-bit external address bus and two byte-select signals \\\\\\\\\\\\\\\"replaced\\\\\\\\\\\\\\\" A0. These 24 lines can therefore reach 16 MB of physical memory with byte resolution. Address storage and computation uses 32 bits internally; however, the 8 high-order address bits are ignored due to the physical lack of device pins. This allows it to run software written for a logically flat 32-bit address space, while accessing only a 24-bit physical address space

Internal registers

The CPU has eight 32-bit general-purpose data registers (D0-D7), and eight address registers (A0-A7). The last address register is the stack pointer, and assemblers accept the label SP as equivalent to A7

Instruction set

The designers attempted to make the assembly language orthogonal. That is, instructions are divided into operations and address modes, and almost all address modes are available for almost all instructions.