6809 instead of the 6502

BruceMcF · Post by **BruceMcF** » Sat Nov 27, 2021 11:28 pm

On 11/27/2021 at 12:42 PM, Edmond D said:

Alas, as a teenager at the time CPU type was a small thing compared to the amount & quality of accessible software (read games?) the platform had.

The functional equivalent of "how many games does it have"?, for this project, would be the supply of skilled assembly language programmers for the processor in question who are willing to donate time to writing the low level Kernel code. Not just the "main guy", but suppose the "main guy" drops out, what are the chances of finding a replacement?

There would be a lot more project risk pursuing a project like this based in a CPU family other than 6502 or z80 families.

However, once the X16 is released, then -- at least as the expansion card interface has been described so far -- it should be possible to do a "Super PET" approach and make a bus mastering 6809 card, for people who want to pursue that.

JazzaMega · Post by **JazzaMega** » Mon Nov 29, 2021 6:35 pm

On 11/27/2021 at 3:41 AM, kelli217 said:

It's certainly in the running for the best 8-bit CPU ever. I don't think an OS like OS-9 could have been written on anything else.

I'd have to interject with the Hitachi 6309, but that's essentially a 6809 clone with additional registers including a 32-bit and 2 16-bit accumulators for 32 and 16-bit math as well as hardware divide, block moves, and bit manipulation instructions.

Moto Rola · Post by **Moto Rola** » Mon Jun 06, 2022 12:43 am

„Computer-Assisted Translation of Programs from 6502 to 6809”

http://www.gtoal.com/vectrex/6502_to_6809/6502to6809.pdf

Moto Rola · Post by **Moto Rola** » Mon Jun 06, 2022 12:50 am

„The beauty of the 6809 is that it can be implanted into existing 6502”

https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-Modern/Archive-Elektor-IDX/IDX/80s/Elektor-1982-06-OCR-Page-0041.pdf

mobluse · Post by **mobluse** » Mon Jun 06, 2022 2:12 pm

Intel 8088 is also an 8-bit CPU since it has an 8-bit data bus. NEC V20 is an enhanced 8088. NEC V30 is an enhanced Intel 8086, but 8086 is a 16-bit CPU. V20 is programmed the same way as V30. V30 is Intel 80186 compatible and has extra instructions and an 8080 mode. I would be rather easy to find V20 developers since they are the same as other x86 developers. V20 and V30 are drop in replacements for 8088 and 8086, respectively.

Moto Rola · Post by **Moto Rola** » Mon Jun 06, 2022 2:42 pm

On 6/6/2022 at 4:12 PM, mobluse said:

Intel 8088 is also an 8-bit CPU since it has an 8-bit data bus. NEC V20 is an enhanced 8088. NEC V30 is an enhanced Intel 8086, but 8086 is a 16-bit CPU. V20 is programmed the same way as V30. V30 is Intel 80186 compatible and has extra instructions and an 8080 mode.

I believe 6502 has been selected just for „sentimental reasons”. Both 6809 and 8088/V20 — with their much „stronger” ML — are far better choice than 6502 for a new „retrosystem” design. Which was „back in the day” selected for its very attractive price, compared to other offers (i8080 was about 8x more expensive in seventies!), not because it was that superior (still it had its advantages, I agree). But today the price isn't that relevant, it's just 3-4 USD less or more. If I'm correct: Z80 is usually cheaper and easier to source than 6502. And of course it offers more.

Even 8085 would be a better fit that 6502, being „almost 16-bit” and having stronger ML (more registers etc.). Motorola 68008 wouldn't be bad as well — from what I see on eBay its price varies from around 4 to 9 EUR. Well, the choice is 6502 with its boring and limited ML. Really a pity.

Scott Robison · Post by **Scott Robison** » Mon Jun 06, 2022 2:46 pm

On 6/6/2022 at 8:42 AM, Moto Rola said:

I believe 6502 has been selected just for „sentimental reasons”. Both 6809 and 8088/V20 — with their much „stronger” ML — are far better choice than 6502 for a new „retrosystem” design. Which was „back in the day” selected for its very attractive price, compared to other offers (i8080 was about 8x more expensive in seventies!), not because it was that superior (still it had its advantages, I agree). But today the price isn't that relevant, it's just 3-4 USD less or more. If I'm correct: Z80 is usually cheaper and easier to source than 6502. And of course it offers more.

While I will not disagree that sentimental reasons definitely came into play for selecting 65C02, it does offer certain advantages to Z80, namely its external data bus access is simpler than Z80. 65C02 generally accesses a byte per cycle, whereas Z80 requires multiple cycles per memory access.

Moto Rola · Post by **Moto Rola** » Mon Jun 06, 2022 2:52 pm

On 6/6/2022 at 4:46 PM, Scott Robison said:

While I will not disagree that sentimental reasons definitely came into play for selecting 65C02, it does offer certain advantages to Z80, namely its external data bus access is simpler than Z80. 65C02 generally accesses a byte per cycle, whereas Z80 requires multiple cycles per memory access.

I agree with you — but that's why Z80 is usually clocked much higher; in general it means faster quartz oscillator is needed. And again: the memory prices aren't that relevant today, as they indeed were „back in the day”. Today requirement for 64 KB (or even 256 KB) of „faster memory” means a few dollars more, not a hundred.

TomXP411 · Post by **TomXP411** » Mon Jun 06, 2022 5:12 pm

On 6/6/2022 at 7:52 AM, Moto Rola said:

I agree with you — but that's why Z80 is usually clocked much higher; in general it means faster quartz oscillator is needed. And again: the memory prices aren't that relevant today, as they indeed were „back in the day”. Today requirement for 64 KB (or even 256 KB) of „faster memory” means a few dollars more, not a hundred.

Yes, it's really a toss-up when it comes to complexity and performance when comparing the Z80 and 6502. The Z80 needs 4 clock ticks to read an instruction, but the Z80 was designed for higher clock speeds. 3.58MHz was very typical, as it's a good clock source for generating TV signals. The VIC and C64, by contrast, used an ≈8MHz pixel clock and divided that by 8 for the CPU.

As to complexity: looking at simple computers like the KIM-1 compared to the Intel SDK-85, it's hard to say which "wins" in terms of the simplest possible computer. In many ways, I prefer the Z80, because it has dedicated I/O commands that don't require memory mapped I/O. This means you can eliminate gaps needed for I/O space and increase the RAM size accordingly. On a CP/M system with a BIOS and drive controller in ROM, you could get away with some large chunks of contiguous RAM. My Altair's CP/M setup has a 63K transient program area, for example, which is hard to get on 6502 systems.

Moto Rola · Post by **Moto Rola** » Mon Jun 06, 2022 6:04 pm

On 6/6/2022 at 7:12 PM, TomXP411 said:

On a CP/M system with a BIOS and drive controller in ROM, you could get away with some large chunks of contiguous RAM. My Altair's CP/M setup has a 63K transient program area, for example, which is hard to get on 6502 systems.

Indeed, that's another thing interesting in case of Z80, V20 or anything else i8080-compatible: the machine could be immediately used under CP/M control, which meanwhile became even open-sourced. Of course it's not contrary to creation of proprietary Commander OS. For 68008 there is CP/M 68k (although I'm not sure, which exactly of the mentioned processors is easy to source today and still will be within next 10÷20 years. Z80 for sure, but no idea about the rest).