Questions tagged [design-choices]
Justifications and trade-offs of historical hardware and software designs, and their possible alternatives.
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Why does the FRE() function in CBM BASIC v2 return negative values?
On Commodore machines featuring CBM BASIC v2, the FRE() function to query the amount of memory available to BASIC returns a negative number when the result exceeds 32767 bytes:
This quirk does not ...
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Why does the FAT file system have separate ‘hidden’ and ‘system’ attributes?
File systems used by DOS and Windows have used file attribute bits as a relatively prominent feature. The first of them, FAT, exposes four attributes to the user: read-only, archive, hidden and ...
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Did they forget to add the physical layout to the USB keyboard standard?
USB keyboards must have been one of the first devices that could be connected to a USB port.
When you are from the US it's possible that you have never faced this problem.
But in all other parts of ...
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8
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Why were nested functions excluded from B and C?
I'm learning C and was curious as to why the language does not allow nested functions.
From what I've read, the lack of nested functions seems to have been a simplification that was inherited from its ...
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Why was there a need for separate I/O address space in addition to a memory address space already?
I was reading through PCI and PCIe configuration access mechanism in Chapter 3 (page 96) of PCIe System Architecture (Mindshare series). As a solution to prevent locking (in case of multiple threads) ...
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Why did old consoles have special RAM dedicated for a specific task?
Even in the PlayStation/Saturn era, they had like little RAM chips which were dedicated to just "sound", or "video", or "general".
Since they still needed to have the RAM ...
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Why didn’t the 1980s micros use MC68010?
These are all legendary 16/32-bit machines that were introduced in 1984 or later that are running the 68000:
Apple Macintosh
Atari ST
Commodore Amiga
Sharp X68000
But the 68010 was already available,...
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1
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Why did the 8085 multiplex data with the low address byte?
Intel’s 8085 used bus multiplexing to stuff more functionality into 40 pins than would otherwise be possible. One of those pins, ALE, signals when the AD0…7 pins are outputting the low byte of the ...
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1
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Why was PETSCII based on an obsolete version of ASCII?
PETSCII (sometimes PETASCII) is the character set developed by Commodore for use in its microcomputers. The first of these, the PET, started to be developed in early 1976. Why, then, did Commodore ...
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7
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Why does the x86 not have an instruction to obtain its instruction pointer?
This has always confused me. Why can you not directly obtain the IP, and instead have to go through some odd assembly hoops such as calling a function whose only purpose is to push its own return ...
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When and why was the question mark chosen to abbreviate PRINT?
In many dialects of BASIC, the PRINT statement can be abbreviated with a single question mark when entering programs or direct-mode commands. So instead of typing PRINT "HELLO, WORLD" you ...
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9
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1
answer
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What is the purpose of the 2x4-bit split of the PCH incrementer?
On this detailed 6502 block diagram that can be found all over the net, the increment logic for the high byte of the program counter is split into two 4-bit parts, with a named PCHC line for the carry ...
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28
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3
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The two types of Return keys on keyboard layouts
Why have these two types of Return keys persisted to this day? A quick look at different keyboards from different keyboard manufacturers from today shows the Shift-style Return seems to have more ...
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7
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Why are symmetric bi-directional communication port cables not always crossover cables?
There are a number of bi-directional communication port standards in which cables are used to connect two identical ports, and usually those connect each pin of one port to the same pin of the other ...
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5
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Why did the stock Amigas not have a battery for keeping the time/date?
The Amiga computers were advanced machines meant to do all sorts of things, including file management. They had a GUI OS (Workbench) and everything right from the very start. They were not some games-...
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4
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Why are the Nintendo 64 memory cards' batteries *soldered* on?
After recently learning that it has a battery inside of it, I unscrewed my Controller Pak (N64 memory card) to put in a new battery. The old one says "98" on it, referring to 1998. So it's ...
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Why are the magnetic floppy disk drives (FDD) heads not frictionless?
After reading this answer to the question How long can a floppy disk spin before wearing out?
... one could see thru the floppy... Not just badly worn, but the magnetic coating was outright polished ...
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2
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How did the 6502 designers intend for the indirect addressing modes to be used?
The 6502 has three indirect addressing modes: Indirect, Indexed Indirect (Indirect,X) and Indirect Indexed (Indirect,Y). Indirect is only used on the JMP instruction, no other. Why was this mode ...
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1
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Why did the NES not allow rotated sprites?
I'm taking a look at the chapter on sprites from a NES programming guide at famicom.party. There is a little table which describes what the different sprite attribute flags do:
Bit #
Purpose
7
Flips ...
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3
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What was the reason that the original Western NES console had a "cartridge bed" that had to be pushed down to play?
In Japan, they had the Famicom. You put in the cartridges on the top, just like with the later SNES and other consoles.
But for Europe and the USA (and the entire "West"), the "NES"...
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Why couldn't early C compilers handle variable declarations between statements?
In modern C, you may place variable declarations between statements:
do_something();
int x;
x = something_else();
However, older C compilers required that variables are declared before all statements:...
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Why did the BIOS load the MBR at 0x7c00?
The IVT is at 0x0000-0x03ff while the BDA is at 0x0400-0x04ff but boot sectors are loaded at 0x7c00. What was at 0x0500-0x7bff that caused this convention? I'm also curious why some MBRs relocate ...
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Why do variable names in BASIC need type suffixes?
The BASIC I'm most familiar with is Atari BASIC since I had an Atari 800 way back when.
The Atari BASIC Source Book includes details about how Atari BASIC maintains variables. There is a Variable Name ...
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2
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Was leaving all xxxxxx11 opcodes unused on the 6502 a deliberate design choice?
The 6502, like many 8-bit processors, has a somewhat arcane opcode-mode restrictions. On most such processors, the restriction is a clear result of trying to pack a lot of instructions into a limited ...
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3
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Why did instruction sets since the late 1970s seemingly stop including an "execute" instruction?
Many mainframe instruction set architectures (ISAs) in the 1960s included an Execute instruction, which would treat data as an instruction.
I haven't found an architecture designed after 1976 which ...
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1
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Why does the ‘Get Next Selector Increment Value’ DPMI call exist?
In DPMI, interrupt 0x31 services 0x0000 and 0x0100 are capable of simultaneously allocating multiple protected-mode selectors in a single call. When that happens, both services return only the first ...
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1
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Why did the Speak & Spell have an alphabetic keyboard?
Many of us remember the original Speak & Spell from 1978:
Image source: Wikipedia
One of the interesting things about this early handheld educational game console is the keyboard. It's not normal!...
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Why were procedure parameter specifications optional in the ALGOL 60 Revised Report?
In Algol 60 procedure declarations, the 'specification' part was optional for by-name parameters. The specification is what gives (loosely speaking) the type of parameter - whether it's real, integer,...
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Why does Pascal have numeric labels?
Pascal was intended, in part, to be a simple language to implement. Some of the design decisions reflecting this are
Declarations/definitions must be given in a strict order (labels, constants, types,...
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What was the first device to "beep" in lieu of tactile feedback to button presses?
I'm splitting this question off of When was beeping invented, in a user interface sense? because I think it's more answerable on its own and I suspect likely to still be computing-related.
At some ...
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When was beeping invented, in a user interface sense?
UPDATE: thanks all, lots of good discussion but I think this question is a bit too vague to be answerable. I'm casting my own close vote against it and will re-ask a more specific one.
Specifically I ...
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Why were single quotes ('…') chosen for characters, and double quotes ("…") for strings?
In C, '' is used to denote a character, while "" is used to denote a string. Why was this syntax chosen?
I tried to research this using Wikipedia’s Timeline of Programming Languages along ...
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Why was `!` chosen for negation?
It seems that the use of the exclamation mark ! to denote negation started with the C programming language (as far as I can tell from my Google research). Nowhere though is mentioned who and why chose ...
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Why did old IBM-PC-compatible computers only have 16 colors available?
In the MS-DOS Editor, the only choices for colors were a collection of 16 colors:
That's 16 colors:
Black
Blue
Green
Cyan
Red
Magenta
Brown
White
Gray
Bright Blue
Bright Green
Bright Cyan
Bright Red
...
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What technical reasons prevented the Win9x Virtual Machine Monitor from running multiple threads simultaneously?
Were there any inherent limitations in the way the VMM for Win9x was designed that prevented it from being able to run threads simultaneously if the underlying hardware had multiple cores/processors? ...
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Why does the 6502 JSR instruction only increment the return address by 2 bytes?
Currently messing with 6502 assembly on a C64, and I don't understand why the JSR instruction is so weird.
According to the instruction table, JSR is a 3-byte instruction and only operates in absolute ...
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What was the last personal computer to have the chips neatly arranged in rows and columns?
Once upon a time, the chips on the circuit boards of personal computers1 were arranged in a tidy grid pattern. Observe the board of the Apple ][+:
Or the Commodore 128:
Sadly, the zen of neat rows ...
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What aspect of portable floating point did Java back down on?
Java was released under the slogan 'write once, run anywhere'; while its adoption was probably more about 'now we have a language that provides garbage collection in a familiar workflow and with a ...
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Why do Amiga Libraries have negative entry points?
As described in these answers, the jump table offsets to functions contained in an AmigaOS library are always negative values. The reason for this is not intuitive to me, but I suspect Carl Sassenrath ...
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2
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Why was the VT05 resolution 72×20?
According to https://en.wikipedia.org/wiki/VT05
the VT05 presented the user with an upper-case only ASCII character display of 20 rows by 72 columns.
At first glance this seems a reasonably natural ...
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Why is the ‘auto’ storage class specifier included in C?
The auto keyword in C seems quite redundant: wherever it makes sense to define a variable with automatic storage duration, it is already the default, so there is no reason to use the keyword. The ...
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5
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Twist in floppy disk cable - hack or intended design?
There are many documents (e.g. this) describing the twist on two-drive floppy disk cable on IBM PC compatibles. While this is not the most proud example of clean hardware design, how much of this was ...
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Why did mainframes have big conspicuous power-off buttons?
Some fascinating stories in this discussion thread.
It starts with discussion about computers overheating, but about halfway through the thread, it switches to discussion of mainframe installations in ...
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Why does -z and -n exist in most shells and /bin/test?
The test command on Unix-like systems provides two special syntax forms for checking whether a string is empty or not:
test -z "$foo" # the length of $foo is zero
test -n "$foo" # ...
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Intel vs Motorola device communication protocols
On this page about "An S-100 68000 CPU Board" it is said:
From a hardware perspective there are some very significant differences how Intel and Motorola CPU's talk to the world. Somewhat ...
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What did Pete Stewart think he knew about efficient implementation of floating point denormals?
The most controversial part of the IEEE 754 floating-point standard is gradual denormals. Typically they trap to software rather than being implemented in hardware. In the common case where a workload ...
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Why did the 6502 handle BCD with a special mode?
The 6502 had special support for BCD arithmetic, because it was widely used in those days; this much, it had in common with other CPUs.
But the 8080 and 6800 implemented this in the form of a 'decimal ...
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5
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Why is the clock frequency of the PS/2 keyboard protocol so high?
The PS/2 keyboard protocol allows the keyboard to generate a clock rate between 10 kHz and 16.7 kHz.
At 11 bits per scancode, 10 kHz is a massive 909 scancodes per second. World-record holder Barbara ...
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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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Why did the Spectrum Next choose Z80 on FPGA? [closed]
I'm curious to understand why the Spectrum Next uses Z80 on an FPGA instead of something like at eZ80? I would have imagined they would choose the eZ80 since it is still a production part. Anyone ...
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