Questions tagged [cpu]

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Why is the processor instruction called "move", not "copy"?

Many processors have an instruction called "move" (sometimes spelled MOV) which copies data from one location (the "source") to another (the "destination") in registers and/or memory. It does not do ...
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54 votes
8 answers
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Why didn't early single-chip CPUs support multiplication instructions

Early single-chip silicon CPUs like the Zilog Z80 or MOS 6502 did not have a multiply instruction at all. Was this because the technology did not exist at the time to implement it, was it too ...
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38 votes
1 answer
3k views

Z80 CPU address lines not stable

I just got a Z84C0020PEC and wired it up to test it, using this circuit: Except that I've added LEDs to A0 through A9. It appears that A0 through A6 operate correctly, but A7 though A9 (I've not ...
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34 votes
4 answers
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How much did IBM save by limiting the PC to 4.77 MHz?

My understanding is the CPU clock speed on the Intel 8088 in the IBM PC was selected as 4.77 MHz to simplify the design of the system. This despite the 8088 coming in two versions - 5 MHz and 8 MHz. ...
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34 votes
2 answers
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Can the two CPUs in a Commodore 128 run at the same time?

The Commodore 128 has two CPUs. One is some variant of the 6502, and the other is a Z80. One CPU is there for compatibility with the Commodore 64 and the other is there presumably to give basic ...
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33 votes
8 answers
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Does the industry continue to produce outdated architecture CPUs with leading-edge process?

Intel has named the i7-8086K in honor of the 8086 processor, though itself it is a 64-bit processor. And we still see in embedded systems or MIL-SPEC platforms there are old CPUs like the 80386 ...
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33 votes
2 answers
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Can an x86 CPU running in real mode be considered to be basically an 8086 CPU?

When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)? Or are there differences between the two?
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33 votes
4 answers
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Why was manual branch suggestion abandoned?

Once pipelined CPUs became common, a common issue arrived as a result of taking the wrong branch of a conditional jump, and thus needing to flush the pipeline. As a result branch prediction mechanisms ...
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33 votes
2 answers
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How did people program for Consoles with multiple CPUs?

I'm specifically interested in the Sega Mega Drive/Genesis, which used a 68000 CPU, but also a Z80, mainly used to control the sound hardware and provide backward compatibility with the Master System. ...
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30 votes
4 answers
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Intel CPU bug in the '90s

My teacher who teaches "Logic" at the university told us a story about Intel processors, which goes: In the '90s Intel had a bug in the calculation of mathematical functions like sin/cos ...
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29 votes
8 answers
10k views

Why did post-8008 CPUs not keep the on-chip stack idea?

Ken Shirriff writes in his blog entry about the 8008: The 8008's seven registers are in the upper right. In the lower right is the address stack, which consists of eight 14-bit address words. ...
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28 votes
3 answers
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Why did Intel abandon unified CPU cache?

When Intel introduced the 80486 in 1989, they included their first on-chip cache, ostensibly to compete better with Motorola, who had been including on-chip caches for 5 years (MC68020, 1984). Unlike ...
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25 votes
4 answers
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Do instruction exercisers exist for 8086 and 68K (and other) CPUs?

For some of retroCPUs there exist exhaustive instruction exercisers, i.e. programs that are capable of catching the implementation errors when run on the emulator under development or on the newly ...
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24 votes
3 answers
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How was the 80186 incompatible with the IBM PC?

According to https://en.wikipedia.org/wiki/Intel_80186 The 80186 would have been a natural successor to the 8086 in personal computers. However, because its integrated hardware was incompatible with ...
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23 votes
7 answers
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What was the last non-monolithic CPU to come to market?

This answer to the question "What was the rationale behind 36 bit computer architectures?" makes the point that early computers were assembled by hand, rather than having central processing ...
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22 votes
2 answers
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What was the last x86 CPU that did not have the x87 floating-point unit built in?

This Wikipedia page says the following: Most x86 processors since the Intel 80486 have had these x87 instructions implemented in the main CPU So the above quote implies that some CPUs that were ...
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21 votes
5 answers
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Will PC-DOS run faster on 4 or 8 core modern machines?

When I run PC-DOS on my 4 core AMD Phenom chip, does it take advantage of the extra parallel CPU's? If not, is there a way to coax DOS to use all available CPU's or does this require specific ...
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21 votes
1 answer
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MOS 8502, just a 6510B?

I'm looking over what little I can find on the 8502, and from what I can see it appears to be a 2 MHz version of the 6510 with an extra I/O pin. Much ado about the speed, but the Atari's 6502B's were ...
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20 votes
7 answers
17k views

What does the "x" in "x86" represent?

I have read the following in the x86 Wikipedia page: The term "x86" came into being because the names of several successors to Intel's 8086 processor end in "86", including the 80186, 80286, 80386 ...
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20 votes
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Why did Socket 3 have more pins than needed for the 486?

Intel's Socket 3, used for 486 processors, was a 19×19 pin grid array socket. However, all compatible processors, to my knowledge, used 17×17 PGA packages. What was the point of the extra pins around ...
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19 votes
4 answers
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Does any computer resemble the model taught in UK secondary education?

In UK secondary education, there's a model called the fetch-execute cycle, which describes how computers work. (See: Isaac CS; Bitesize GCSE, Higher; Teach CS.) As I understand it: The processor has ...
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19 votes
4 answers
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Were there any 8-bit CPUs with 24-bit addressing?

Or was that something that didn't appear until later CPUs (around the time of the 286 maybe)? Also, how would I go about researching this on my own? It's not exactly something I can look up on ...
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19 votes
1 answer
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What are the screws for on the UltraSPARC?

I fail to grasp what can be screwed on them, can you explain what are these for?
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17 votes
2 answers
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DEC Alpha: why no 8/16-bit load/stores?

The first version of the DEC Alpha had no load/store instructions for 8 or 16-bit values; if you wanted to deal with data of such sizes, you had to do it by shifting and masking values in registers as ...
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16 votes
5 answers
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Does the Intel 8086 CPU have user mode and kernel mode?

Does the Intel 8086 CPU have user mode and kernel mode as modern CPUs do? and if it doesn't have user mode and kernel mode, does that mean that any user program written for the Intel 8086 CPU could do ...
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16 votes
2 answers
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Why did some CPUs use two Read/Write lines, and others just one?

Many 8-bit processors, such as Motorola's 6800 and MOS Technology's 6502 make use of a single pin to indicate to the rest of the system whether the CPU wishes to read from or write to a memory ...
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16 votes
1 answer
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How do accelerators and CPU cards work on the Apple II?

An Amiga 1200 exposes the entire CPU bus on the expansion port, so that an accelerator only needs to assert BR which causes the onboard CPU to stop all computation and electrically disconnect from the ...
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14 votes
7 answers
5k views

Does the Intel 8085 CPU use real memory addresses?

The Intel 8086 CPU uses memory segmentation, which means that when, for example, you write the value 123 to the memory address 1001, the memory address 1001 will actually get converted first into ...
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14 votes
5 answers
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Has there ever been a instruction set architecture that did not require instruction decoding at all?

I am studying basic principles of instruction set architectures and am considering what it would take to not have any instruction decoding at all. I.e., all the control lines of the computer would be ...
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14 votes
3 answers
3k views

When was the 6502 second sourced?

According to https://en.wikipedia.org/wiki/Second_source MOS Technology licensed Rockwell and Synertek to second-source the 6502 microprocessor and its support components. This makes sense; the 6502 ...
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13 votes
1 answer
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Intel processor transistor type evolution

The Intel 4004 used MOS (metal–oxide–semiconductor) transistors. What has been the transistor types used in Intel processors onwards from the 4004 to 8085 to the x86 family of instruction set ...
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13 votes
3 answers
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What was the first CPU with exposed pipeline?

Quoting from Programming for Performance exercise: early versions of the MIPS processor had an "exposed pipeline" (that is, the assembly language programmer needed to know the latencies of ...
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13 votes
4 answers
2k views

How did 2-chip CPUs work?

The 1970s saw a big transition from CPUs built from thousands of discrete components, to CPUs implemented on a single chip, with the occasional use of bit-slice components along the way. There were, ...
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13 votes
2 answers
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Resources about 1980s GPCPU video controller designs

Reading about the Fujitsu Micro 7, I found out that it used a pair of 6809 microprocessors: one as main CPU and one as graphics processor. That made me remember that when running old arcade games with ...
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12 votes
6 answers
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Did any RISC CPU ever take more than one clock cycle per instruction?

Classic RISC CPUs like ARM and MIPS basically offer the trade-off: simple instruction set, but instructions execute in one cycle for good overall performance. (It gets more complicated in later times, ...
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12 votes
2 answers
3k views

How did the CP1600 CPU supposedly make looping faster?

I came across an Intellivision in a box of stuff. When I looked it up on Wikipedia, it said that it uses a CP1600 that is based off a PDP-11. There's a weird entry on the wiki page: CP1600 did not ...
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12 votes
3 answers
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What was the first microprocessor to support full virtualization?

Virtual memory, which allows an operating system to run several machine code programs isolated from each other, came to the desktop during the eighties. But full virtualization, which lets the ...
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11 votes
2 answers
674 views

Performance of the Rekursiv

I happened to find out about the Rekursiv today. Rekursiv is a processor that attempted to implement OOP concepts directly at the hardware level. Since it never got fully developed, I wonder what ...
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11 votes
1 answer
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How did the Zip Chip and RocketChip accelerators work for the Apple II?

Though there is already a similar question on the site, this one is more specific to these chips and their technical implementation. There were two accelerators for the Apple II series that were drop-...
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9 votes
6 answers
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Is there any music source code for sound chips? [closed]

I’ll clarify what I mean. The sound chip (c64's SID, spectrum's Yamaha, etc.) is connected either to the input / output port, which will be reserved for the sound chip, or directly to the CPU data bus....
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9 votes
3 answers
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Did multiplexed address/data lines make memory access slower?

Looking at the pinout of e.g. the 8088, it multiplexes the data lines onto eight of the address lines; presumably the designers judged that being able to squeeze the chip into a 40-pin package, would ...
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9 votes
4 answers
1k views

Why did later CPUs use microcode instead of PLA's?

If you look at a die photo of a 6502, about forty percent of the chip is taken up by what's obviously microcode, both by its regular structure and by the obvious need for such from the instruction set,...
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8 votes
2 answers
789 views

Instruction set support for multiplication with a constant

Before integer multipliers in silicon, several cpus had some support for multiplication. For instance SPARCv7 has the MULScc multiply-step instruction (several other cpus also have this). As far as ...
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8 votes
1 answer
266 views

How does the VIC-II/CPU memory access work on the C64?

As a programmer I knew that on the C64 the CPU got the bus in the high phase of PHI2 and the VIC-II got it in the low phase and also stole extra high phases when required. However, it wasn't until ...
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7 votes
5 answers
743 views

Could the Intel 8086 CPU have many segments in memory of the same type?

The Intel 8086 CPU could address up to 1 MB of memory using segmentation, and this CPU have 4 segment registers, which are CS and SS and DS and ES. Each segment in memory can have a maximum size of ...
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7 votes
2 answers
800 views

What was the first microprocessor to overlap loads with ALU ops?

Modern out of order CPUs can do all sorts of things in parallel, having not only multiple functional units, but a lot of logic to check at runtime exactly which instructions really depend on others, ...
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7 votes
1 answer
2k views

Did any CPU ever expose load delays?

There have been CPUs with exposed branch delays, such as early MIPS: What was the first CPU with exposed pipeline? (Later MIPS kept the delay slots from the early MIPS, though by that time, it wasn't ...
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7 votes
2 answers
305 views

What is the relation between external clock and internal states in the 68000?

(I'm assuming a memory cycle of 500 ns, without wait states.) According to the 68000 bus diagram, there are 4 CPU cycles for a memory cycle, so an external frequency of 8 MHz. However, things are ...
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7 votes
1 answer
302 views

What CPU architecture was first to implement 'inverted borrow' carry flag during subtractions?

Background In two's complement arithmetics, if one wants to calculate a subtraction having only an adder that calculates {cout,result}=full_adder(arg1,arg2,cin), where cin and cout are incoming and ...
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6 votes
2 answers
863 views

GPU chip implementation on CPU - will it be faster? [closed]

So, modern video cards include two devices - a video signal generator and a graphic accelerator. The first of them converts data from video memory into MDA / CGA / VGA / RCA / DVI / HDMI / etc format....
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