## Microprocessors Lecture 18

### Microcontrollers (Single-Chip computers)

Microprocessors from the major manufacturers possess a family resemblance.

Motorola entered the 8-bit microprocessor market with the 6800 processor.

6800 | Basic microprocessor
6802 | on-chip clock 128 bytes RAM a 3-chip microcontroller is possible
6809 | compatible with 6800 source code software is easily converted some 16-bit instructions MUL instruction 2 x 16-bit index registers 2 x 16-bit stack pointers autoincrement indexed addressing variety of offsets for indexed addressing
6801 | microcontroller 2k ROM, 128 bytes RAM 31 parallel, 3 serial I/O 2 x 16-bit timers can access up to 64k bytes externally has 16-bit instructions MUL instruction

Microcontrollers, such as the 6801, result in a very low chip count since the CPU, RAM, ROM and I/O are on the one device.

Mention should also be made of the 8048 family of devices as they are quite common.

8048 | 1K ROM, 64 bytes RAM 3 x 8-bit parallel ports, timer (an earlier device than the 6801)

### 16/32 bit processors

From the 8-bit processors, most manufacturers introduced 16-bit devices.

8085 | -> 8086 | Intel
Z80 | -> Z8000 | Zilog
6800 | -> 68000 | Motorola

### Advantages of 16-bit processors

Since the data bus is 16-bits wide, 2 bytes are read or written to each memory cycle. Therefore data transfers are faster for a given bus speed.

As technical designs moved forward, extra logic was possible on-chip. This was used to provide extra registers, instruction logic and interrupt and bus handling.

In the same way, more address lines were added to the processor, so that 16 Mbyte address space became common, compared to the 64 kbyte limit of 8-bit processors.

Architectures were developed to make register to register transfers more common than in 8-bit designs.

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