VDict - Definition of Motorola 6809

Computing (FOLDOC) dictionary

Motorola 6809

The 6809 was a major advance over both its predecessor, the

Motorola 6800 and also over the 6502. The 6809 had two 8-bit

accumulators, rather than one in the 6502, and could

combine them into a single 16-bit register. It also featured

two index registers and two stack pointers, which allowed

for some very advanced addressing modes. The 6809 was

source compatible with the 6800, even though the 6800 had 78

instructions and the 6809 only had around 59 (including a

SEX instruction). Some instructions were replaced by more

general ones which the assembler would translate, and some

were even replaced by addressing modes.

Other features were one of the first multiplication

instructions of the time, 16-bit arithmetic and a special fast

interrupt. But it was also highly optimised, gaining up to

five times the speed of the 6800 series CPU. Like the 6800,

it included the undocumented HCF (Halt and Catch Fire) bus

test instruction.

The Hitachi 6309 was a version with extra registers. The

6809 was used in the UK "Dragon 32" personal computer and

was followed by the Motorola 68000.