Questions tagged [6809]

For questions regarding the Motorola 6809 microprocessor.

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What does the DAA instruction do?

The 6502 has the decimal mode, which causes the ALU to always output valid BCD if it's given valid BCD. Of course, you can turn it off, as any sane person always does. I was just looking at the ...
Omar and Lorraine's user avatar
18 votes
3 answers
3k views

Are the 6809 and 6809E different from a programmer's point of view?

I can see the pinouts of these two chips are utterly different. Apparently the 'E needs an external clock as well. This question isn't so much about any of those hardware related differences. This ...
Omar and Lorraine's user avatar
2 votes
2 answers
526 views

Where can I find the Motorola Application Note AN-754?

Urgently need a copy of Motorola Application Note AN-754 "Device Operation and System Implementation of the Asynchronous Serial Interface Adapter". I have two boards from college days (1979) ...
Joe Blackburn's user avatar
13 votes
2 answers
1k views

How did the 6809 achieve source-code compatibility with the 6800?

The 6809 has 59 instructions, as compared with 78 on the 6800. So far as I can see, the 6809 lacks instructions that the 6800 has, such as aba, eorb and the transfer instructions (tba, tap and the ...
Omar and Lorraine's user avatar
13 votes
3 answers
2k views

How to use rounding with 6809 multiplication

The MC6809 is unusual for an 8-bit processor of its era in having a hardware multiply instruction. MUL multiples the 8-bit unsigned integer in the A register with the 8-bit unsigned integer in the B ...
Wayne Conrad's user avatar
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3 votes
1 answer
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Microware OS-9 assembler warning with no message

My 6809 assembly language program for the coco3 is causing the assembler to issue a warning with no message. How can I find out why the warning is being issued? From the assembly listing, showing ...
Wayne Conrad's user avatar
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15 votes
2 answers
2k views

Why did MC6809(E) require two-phase clock consisting of two square waves shifted by 90°?

Usually when talking about two-phase clocks in early CPUs, one means this type of clock, that consists of two non-overlapped pulses. Examples are MC6800 clock (page 3) or i8080 clock (page 6). Many ...
lvd's user avatar
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11 votes
1 answer
641 views

Can anyone identify this Parallel to Serial Interface Board?

I just spent an enjoyable day at the local ham fest with my father - digging through boxes of other peoples junk. I'm a sucker for weird electronics, especially when it's in the free pile, and so I ...
Geo...'s user avatar
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21 votes
2 answers
1k views

Fate of mystery Motorola RMS graphics chipset (68486 + 68487)

I recently learned that mid 1980s, Motorola was working on a discrete graphics chipset that would be a great leap forward from its 6847 Video Display Generator and 6845 display controller. It was ...
Fedor Alexander Steeman's user avatar
17 votes
2 answers
5k views

Why was the 6809 so expensive?

The 6809 was released in 1978, but looking at the usual source for price quotes for old computer components, Byte magazine, I cannot find any quotes for 1979. December 1980 lists it at $38, compared ...
rwallace's user avatar
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9 votes
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How useful were the 6809/6309's indirect addressing modes?

Much like the VAX, the 6809 and 6309 have a preponderance of indirect addressing modes (in VAX parlance, "deferred"), though neither go so far as to include a VAX-like doubly-indirect mode. To what ...
junius's user avatar
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6 votes
2 answers
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Hardware multipliers on classic CPUs

Can anyone offer some back-of-envelope sort of estimates of what sort of gate count was needed to add a multiplier to 8-bit CPUs? I ask because I find it a bit odd that the 6809 had one, but none of ...
Maury Markowitz's user avatar
13 votes
4 answers
2k views

Motorola 6809 two stacks design

I'm an assembly newbie, but I'm curious about why the designers of the 6809 choose to use 2 stack in the microprocessor architecture. What is their original intent? The 6809 Wikipedia page says "...
Biff Iam's user avatar
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48 votes
10 answers
16k views

What limited the use of the 6809 CPU in personal computers?

Why did manufacturers of home computers avoid using the 6809 CPU? I realize that the Z80 and 6502 had a 3- or 4-year head start in availability. But once it did become available in 1978, I don't ...
RichF's user avatar
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6 votes
1 answer
105 views

HD63B03R timer interrupt frequency

I am trying to maintain an old procuct for my employer, based on an HD63B03R which is a 6800-style micro. The crystal on the PCB generates a 4.75MHz clock. I believe from the data sheets found on t'...
GEphil's user avatar
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13 votes
1 answer
3k views

Calculating the time for a CPU cycle

As a learning exercise, I'm writing an emulator for the 6809 CPU. I'd like to simulate the speed of the CPU. I know the Motorola 6809E runs at 0.895 MHz and 1.79 MHz on the Color Computer 3. How do I ...
8BitCoder's user avatar
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17 votes
1 answer
1k views

Motorola 6809 Stack Blasting

I've seen several mentions of a "Stack Blasting" technique for copying memory quickly on the 6809. What is it? How does it work?
tlindner's user avatar
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40 votes
3 answers
3k views

Advantages of a Hitachi HD6309 versus a plain Motorola MC6809

Hitachi was a second source for the Motorola 6809 Micro Processor. The 6309 had many secret features. What advantages does the HD6309 have over the MC6809?
tlindner's user avatar
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