36

The premise in the question is incorrect. There were such chips. The question also fails to allow for the way that the silicon-chip industry developed. Moore's Law basically said that every 18 months, it was possible to build chips with twice as many transistors for the same amount of money. The 6502 (1975) is a mid-1970s design. In the '70s it cost a lot to ...


26

Each of the 68K series CPUs had dedicated address-generation hardware which was wired more directly to the A registers and had only limited access to the D registers. Conversely, the main ALU was more directly wired to the D registers than the A registers. It thus became a performance enhancement, allowing the main ALU and the addressing logic to operate ...


16

The 65816 was close to the bare minimum of a 16 bit processor. It was primarily used where compatibility with existing 6502 code was needed, such as with the Apple IIgs. It was also used where the designers of a new 16-bit system were already familiar with 6502. This is probably why the SNES has the 65816, given the NES had the 6502. By the time the 32 ...


12

Swapping byte lanes on the physical bus would, in any case, only have an effect on naturally aligned data in memory, which happened to be the same width as the bus. Swapping the lanes of a 16-bit bus doesn't solve the problem for 32-bit data, nor on a 32-bit bus for 64-bit or 80-bit data (the latter being associated with floating point). So that is not a ...


9

I don't understand why western design centre made the 65816 a 16bit upgrade to the 6502 but commodore semiconductor group/MOS technology didn't make their own variant For one, the 65816 is only a 16 bit CPU in a very restricted way. All external transfers are still 8 bit wide and address expansion is rather clumsy. The main improvement wider architectures ...


8

Yes, you can use -phxass option (name of the old Amiga native assembler by Phx/Frank Wille who is also the author of the Motorola syntax module) vasm -phxass -m68020 -kick1hunks -Fhunkexe -o test.exe main.asm From the documentation: ‘-phxass’ PhxAss-compatibility mode. Only directives known to PhxAss are recognized. Enables the following features: Enable ...


6

Adding command line option -spaces will do the trick by allowing spaces within operands. This means of course that comments now must be separated by ; from any instruction or directrive. Unlike with -phxass (which includes -spaces), there are no other side effects. The option is only available with Motorola syntax (vasmm68k_mot). On a sidenote, it may be ...


6

Since you mentioned the Amiga in your question, it ought to be relevant as to how this problem was solved efficiently on that system. As I understand, the "glue" logic for the Bridgeboards consisted mainly of 128KiB of dual-ported RAM accessible by both the Intel CPU on the Bridgeboard and the Amiga's 68K. This would be sufficiently large to buffer ...


5

The primary advantage of the 68000 segregating address and data registers is the ability to have many instructions use three-bit register-select fields, thus saving opcode space compared with using four-bit fields. IMHO, the ARM Thumb instruction set and derivatives could have benefited from employing such a concept, since otherwise the upper registers end ...


5

This take a little more circuitry than the OP, in that the byte-swapper has to make use of the Function Code bus lines (FC0, FC1 and FC2) to distinguish between data transfers and instruction fetches. Bytes should only be swapped for data transfers; applying byte-swapping to instructions will wreck the execution of software.


5

The job is non-trivial, but I don't think it would be hard for someone 'experienced in the art'. 256k or even 512k is not a lot of code, and the ROMs are highly structured with a well-defined interface. I suspect it would be easier than cracking some of the more sophisticated copy-protection schemes used in Amiga games. In some respects ROM code is easier ...


4

To add to the other answers, the 65xx family design, with just a few on-chip registers, made sense when transistors were expensive, and memory accesses were cheap. That allowed using the zero page as, essentially, a large and flexible register set. A quick look at the WDC 65C832 datasheet reveals that it sticks to that philosophy, presumably because the ...


4

Because binary compatibility in most cases is overrated and not worth compromising a design to maintain. The only reason to extend a chip family like that is to maintain binary compatibility. In contrast, "reinventing the wheel" with a new design empowers the designers embrace everything the field has gained over time. If anything, Intel is the ...


3

[While having a twist in its own, I got a feeling this had be answered already some time ago, in context about the ways to mark bus cycles on 8 bit CPU, but can't find it right now] To start with, the page mentioned compared kind of Apples and Oranges, as it's about Intel 8080 vs. Motorola 68000 (*1). These were complete different CPUs designed at different ...


3

How does this work from the endian-ness perspective? Endianness is for most parts a software issue. Hardware, especially memory is either agnostic to the way a word ist stored, or doesn't have any idea about units wider than a byte at all(*1). Basically all I/O on S100 is 8 bit wide - after all, continuing the use of existing peripherals was a main point of ...


2

A long time ago I worked on an Alpha Microsystems computer which was S-100 based little-endian, and while I was there we upgraded to a 68000-based system that had been wired (I don't know the details) to behave as if it was a little-endian system. So I can't give you details, but it is possible.


2

Buses like S-100 are "endianness-agnostic". They do not care about data and its order, they just transport desired content as CPU (or "master") asks. Endianness is "the CPU thing".


2

According to the project page on GitHub for the Open68000Relocator board: The board has been tested and proved to be working perfectly with a TF530 card running at 50 MHz. So, if you can use the same swapping of the 68k orientation in the CDTV, then this re-locator would probably have a good chance of working. Likewise, some minor variation that does not ...


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