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60 votes

Why were there no 32-bit versions of 65xx CPUs, or 64-bit versions of m68k CPUs?

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, ...
Liam Proven's user avatar
  • 1,255
59 votes

Why did the Motorola 68000 processor family fall out of use in personal computers in the 21st century?

The 68k family instruction set, as elegant as it appeared to the casual assembler programmer (been there), had several flaws that made it very difficult to get fast in hardware. Out of order or ...
Patrick Schlüter's user avatar
56 votes

Why weren't 80s arcade games programmed in C?

I think the question I would ask is why would you program arcade gamers in C back in the 80's. Firstly, C was not nearly as popular in the world of microprocessor programming as you might imagine back ...
JeremyP's user avatar
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41 votes

How did the Apple II forward binary instructions to the Z80 software card with CPM?

They both shared the same memory so it didn't really forward instructions. The Z80 card stopped the 6502 running using the DMA signals and the system swapped between the two by writing to $CN00 where ...
PeterI's user avatar
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41 votes
Accepted

Why did the Motorola 68000 processor family fall out of use in personal computers in the 21st century?

The Apple-IBM-Motorola alliance was created in 1991 to compete with the Windows/Intel market. Its main successes were the creation of the PowerPC instruction set, derived from IBM's POWER architecture,...
John Dallman's user avatar
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36 votes
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Replacing 80286 with 68000

No, there is no simple one-to-one mapping for the pins. (Bolded signal names will be active-low.) For example, while the 286 has two physical pins for interrupts (INTR and NMI), 68000 has three (IPL0,...
telcoM's user avatar
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35 votes
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What makes MOVEQ quicker than a normal MOVE in 68000 assembly?

The MOVE immediate instruction takes 8 cycles in byte and word modes. There are two memory reads, one for the instruction and one for the immediate value. The MOVEQ instruction encodes the immediate ...
user's user avatar
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31 votes

Why didn’t the 1980s micros use MC68010?

Available isn't the same as in volume production, where big enough computer manufacturers could negotiate a better price in higher volumes. Both the Macintosh and the Amiga projects started out with ...
hotpaw2's user avatar
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29 votes

68000 and memory access speed

If you look at the datasheet of a typical DRAM chip of this era, say the Mostek 4116, it indeed has a cycle time of 375ns, so you can't access it at more than 2.6 MHz. But don't confuse the clock ...
dirkt's user avatar
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29 votes
Accepted

Did the 68000 separate A/D registers save circuitry?

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 ...
Chromatix's user avatar
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27 votes
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How did the original Apple Macintosh and Atari ST use protected mode?

The (plain) 68k never had anything directly comparable to the Intel x86 range's Protected Mode. When Intel introduced the Protected mode (PM) to its x86 range of CPUs, this lifted a number of ...
tofro's user avatar
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27 votes

Why didn’t the 1980s micros use MC68010?

While "why hasn't X been used" questions are inherently weak, still, one may consider some points: While introduced in 1982 the 68010 wasn't available before mid 1983. Too late for any of ...
Raffzahn's user avatar
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26 votes
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Why is the Amiga ROM at a high memory location, and RAM in low memory?

It was part of the 68000 system architecture in which all the interrupt vectors are low in the memory map. The first 1024 bytes are reserved for these vectors and if a program / os need to change ...
jim's user avatar
  • 386
25 votes

Why is the Amiga ROM at a high memory location, and RAM in low memory?

All of the 68k-based computers (Amiga, Atari ST and Sinclair QL, as well as the classic Macintosh) went to market in a rush. And all of them went to market before the OS (and, thus, the ROMs) were ...
tofro's user avatar
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24 votes

Why were there no 32-bit versions of 65xx CPUs, or 64-bit versions of m68k CPUs?

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 ...
RETRAC's user avatar
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24 votes

How much space did the 68000 registers take up?

The Motorola 68000 has 16 (somewhat) general-purpose registers of 32 bits each Well, not really; the 68k ISA does not feature a single set of 16 General Purpose Registers (GPRs) but two sets of 8 ...
Raffzahn's user avatar
  • 225k
23 votes

How did the Sun-1 handle page faults despite having the original 68000 processor?

I haven't looked at the relevant versions of SunOS, so I can't say for sure which method they chose, but I'm aware of three methods for dealing with this problem. The first method uses two CPUs. You ...
Jerry Coffin's user avatar
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21 votes
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How much space did the 68000 registers take up?

In the photo you link, they are near the bottom, part of the address execution unit, and data execution unit. In this cropped section from a higher resolution image (linked in the comments), you can ...
RETRAC's user avatar
  • 13.8k
20 votes

Replacing 80286 with 68000

Most obvious question first: why not puting itn on a ISA Card and take over the bus instead? Given, there would be still some work to be done after asking for DMA and pulling /MASTER, but way less ...
Raffzahn's user avatar
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20 votes
Accepted

What limited the use of the Z8000 (vs. 68K and 8086) CPU for 16-bit computers?

The Zilog Z8000, Motorola 68000, and Intel 8086 all arrived at roughly the same time-frame and each represented the new 16-bit architecture of their respective CPU designers. They differed markedly in ...
Brian H's user avatar
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19 votes

Why did the Motorola 68000 processor family fall out of use in personal computers in the 21st century?

Motorola stopped investing in MC68000 family when everyone thought that RISC was the future and that CISC CPUs would be soon non competitive. So it switched to PowerPCs. Even Intel thought this and ...
Grabul's user avatar
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17 votes
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What was Burst Mode on the 68030 and why didn't the A2630 support it?

I'm curious what burst mode was on the 68030 and It's an extension to the 68020 bus protocol offered by the 68030. By default, all read access on a 68k takes two cycles. Cache Burst uses the same two-...
Raffzahn's user avatar
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16 votes
Accepted

Can a retro-computer be a useful way to learn computer-architecture fundamentals?

The main benefit for learning computer system architecture with a retrocomputer, compared to a modern computer, is accessibility of the hardware. Retrocomputers are much simpler than modern systems in ...
Brian H's user avatar
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16 votes
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Why was CPU32 core created, and how is it different from 680x0 CPU cores?

I can suggest why it was created. The Wikipedia page for CPU32 says: The instruction set of the CPU32 core is similar to the 68020 without bitfield instructions, and with a few instructions unique ...
John Dallman's user avatar
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15 votes
Accepted

How did the Apple II forward binary instructions to the Z80 software card with CPM?

TL;DR: this longish answer address the "mystic" property of the question; i.e., the sense of wonder about how this could be possible; not the actual workings of the specific components. The gory ...
AnoE's user avatar
  • 1,569
15 votes
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Did Nintendo change its mind about 68000 SNES?

Did Nintendo really change their mind about using the 68000? Hard to say, as these decisions were never public. If so, how does this square with that CPU being so cheap even two years before the ...
Raffzahn's user avatar
  • 225k
15 votes

Why weren't 80s arcade games programmed in C?

Many of the people who programmed arcade games got their start in an era where C compilers existed, but would have been expensive and not very convenient to use. Later in the 1980s, compilers reached ...
supercat's user avatar
  • 37.1k
14 votes

Why didn’t the 1980s micros use MC68010?

The main thing which was absent in the 68000 but added in the 68010 was the ability to have an instruction execute until it attempts to access a protected region of memory, trigger a page fault, have ...
supercat's user avatar
  • 37.1k
13 votes

What makes MOVEQ quicker than a normal MOVE in 68000 assembly?

To give the exact cycle-by-cycle breakdown: MOVEQ is a one word instruction so will nominally perform in four cycles; in practice it can occur immediately following operation decoding because all ...
Tommy's user avatar
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