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.

6800Basic microprocessor
6802on-chip clock
128 bytes RAM
a 3-chip microcontroller is possible
6809compatible 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
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.

80481K 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-> 8086Intel
Z80-> Z8000Zilog
6800-> 68000Motorola

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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