How similar were the MC6800 and MOS 6502?

Question

When reading about the history of the 6502 CPU, it is regularly pointed out that Chuck Peddle had previously worked on the MC6800 CPU at Motorola. As well, it is stated that the goal when creating the chip was to produce a cheaper alternative to the 6800, and that was achieved when you compare that the 6800 cost $300 versus the 6502's $20 introductory price. On further reading, there are also mentions of the rare 6501 chip which was actually pin compatible with the 6800 and thus presumably could be a drop-in replacement, though not binary compatible.

So the question is how similar were the two competing microprocessors? Was the performance the same? How [dis]similar were the instruction sets? As well, could a 6501 be dropped into a 6800 socket and work with an existing motherboard despite not being binary compatible?

Answer 1

The 6500 is based on 6800 but cut down on everywhere when possible.

They both are 8-bit micros and they tend to be pretty similar. 8-bit data bus, mostly 16-bit address bus for addressing 64 kilobytes, and no separate IO space, everything is memory mapped.

The 6800 has two 8-bit accumulator registers A and B, sometimes used together as a 16-bit register D. It also has a single 16-bit index register IX or X and a 16-bit stack pointer.

The 6500 has only one 8-bit accumulator, two index registers X and Y but they are both 8-bit registers, and a 8-bit stack pointer.

Both CPUs have addressing mode to access the first 256 bytes (zero page) of RAM directly with a single address byte, faster than when providing two bytes for full 16-bit addressing.

As the 6500 has no 16-bit index registers, there needs to be an addressing mode which points into first 256 bytes from where to fetch a two-byte 16-bit address.

So that typically means that operations on 6500 take more opcodes and memory accesses to perform same as the 6800 and are thus slower, even if the opcodes to load or store or manipulate data are rather similar like on any 8-bit CPU.

So the use of zero page is mandatory on 6500, and on 6500 the stack must exist on the next 256 bytes, i.e. page 1, as the stack pointer high byte is always fixed to 0x01.

Which means that to replace the 6800 with the plug-in compatible 6501 version, the hardware must have some amount of RAM implemented on first 512 bytes for zero page variables and stack, or it has no way of working. For example, Atari 2600 puts 128 bytes of RAM and it can be accessed both from as zero page at addresses 0x0080 to 0x00ff and as stack page addesses 0x0180 to 0x01ff.

The 6800 system can have RAM anywhere as the variables can be stored anywhere and the stack too, so use of zero page and RAM at beginning is not mandatory. Many systems however do start RAM at 0x0000.

They both boot from the end of memory space so in both systems there would be ROM at 0xfff0..0xffff area for reset and interrupt vectors.

Answer 2

Its a matter of opinion, but in mine, from a programmer's perspective they're actually quite different. They're both 8-bit "accumulator" microprocessors, but that's about the entire overlap.

• 6800 has two 8-bit accumulators (so more like registers), while 6502 has one (true accumulator)
• 6800 has one 16 bit index register, while 6502 has 2 8-bit index registers
• 6800 is big-endian, while 6502 is little endian
• 6800 has a 16-bit stack pointer, while 6502 has an 8-bit one.
• 6502 has these double-indirect zero-page addressing modes (so the instruction has an address in zero page, which is fetched to get the address to fetch the actual operand from)

The last in particular makes programming the 6502 very different from programming the 6800

Answer 3

I would think any answer to this question will be, as lvd already noted, quite tied to the use case looked upon if not highly opinionated anyway. So I would like to underline the questions made with some general facts(*1), anyone can use to check the opinionated parts.

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So the question is how similar were the two competing microprocessors?

Well, both are

• Accumulator based designs (like most others of the time),
• Offer a rich range of addressing modes (including indexing),
• Use memory mapped I/O,
• Use similar clock per instruction,
• Operate on a similar bus design,
• Using a similar memory access cycle,
• Available in similar speed grades

Was the performance the same?

CPU's with the same basic design, similar memory access cycle and similar cycles per instruction usually end at the same over all speed range. No matter if 6500, 6800 or others like the 2650 (*2).

How [dis]similar were the instruction sets?

Usually an Assembly programmer of one would be able to get familiar with the other within hours, as they are both memory dependent accumulator machines. Including the same basic instruction set using the same or quite close mnemonics.

The only notable difference are complex addressing modes which are handled different - and a few 16 bit instructions the 6800 offered. Nothing really hard to come by.

Where the 6800 is more of a straight school book CPU, the 6500 shows traits of optimization toward embedded use. Thus being less great to code for more dynamic tasks. But again, not a major difference for real world usage.

As well, could a 6501 be dropped into a 6800 socket and work with an existing motherboard despite not being binary compatible?

That was what it was meant for: a 100% compatible drop in replacement due using the same bus interface - only different software.

This is further supported by both using the same basic memory layout of RAM (and IO) at bottom and ROM at top of the address space. So unless the 'motherboard' is was a very special (and very early) embedded design (*3), chances are good. Even more if we, as motherboard suggests, talk about a more generic, PC like design.

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*1 - It does not make any sense at all to compare on an instruction base or instruction sequences, as any noted difference will depend on the task to be performed and the selection used. Like having a multiply instruction may be yield superior local performance but not really matter over all.

*2 - Not to mention that for most part CPU speed was (and still is) memory dependent, so any CPU using a certain speed grade of memory will yield broadly the same real world performance, including complete different architectures - that is unless a lot of additional logic is thrown at, allowing very complex instructions.

*3 - Something one would have used an 6801 or 6802 anyway. They use a different pinout/bus interface, with no direct replacement offered by MOS.

Answer 4

Every attempt to answer to "how similar" or "was the performance the same" would either be opinionated or (for the last one) depend on specific case.

However, the last question has definite answer: yes, it could, at is was built that way exactly. 6501 was designed to work in any existing 6800 installation, provided the ROM has new, 6502-compatible binary.

Answer 5

The MC6800 had a dual phase clock with tight requirements (both timing and voltage swing) that required a tricky design and decent test equipment to verify. They later came out with the MC6875 clock driver chip. The MCU was introduced at something like $395 USD, if memory serves. The clock driver chip was also not cheap. For example, a clock driver design from this source:

Kind of a wonky design that depends on gate delays for timing.

The 6502 was introduced at a (relative) bargain $25 USD and worked with a crystal or single phase clock. The main enhancement that I recall was the two index registers that made a lot of assembly language routines much more elegant.

My impression at the time, right or wrong, was that the 6502 architectural decisions were pragmatic based on actual programming patterns, and the MC6800 ones were more ideologically driven.

Answer 6

While the 6800 and the 6502 are quite similar at a basic technology level, their different levels of commercial success suggest that the better-optimised ISA of the 6502 was an advantage.

The 6800 is history, as is the 6809. The 68000 survives in modified form as an embedded processor. That was always the 6502's target market, and it's still alive there. Western Design Center claim that hundreds of millions are still produced annually, embedded in SoCs as a simple and cheap CPU that's good enough for many jobs.

That success is a testament to the power of pricing, a much-neglected part of marketing a product, and the benefits of focussing on and developing a product as technology advances.

As well as that, its widespread use as a home computer CPU in the seventies and early eighties helped. That meant there were a lot of people who knew its assembly language. Its primary qualification for home computers was its low price, but its ease of interfacing, simple bus cycle and effectiveness at running interpreters definitely helped.