45

TL;DR Because it needs the least chip count and thus makes it the cheapest. It's a Sinclair. Full Story: The Sinclair ZX80 used a Z80A running at 3.25 MHz. But this chip was rated for 4 MHz. Why was it run below rated speed? And the chip would have run for sure at more than 4 MHz. A computer design isn't about what a chip is rated, but what it's for. ...


17

Hardware of this sort has to be able to cope with the worst-case scenario in any given dot-clock cycle. So it has to look at the top layer pixel, determine whether that is transparent, and if so go down to the next layer and repeat. Only when it finds an opaque pixel (which may be the background) can it determine the colour to drive the video output with. ...


14

They did, with the A600. But in true late-stage Commodore fashion, they screwed it up and made it more expensive. To cost-reduce an A500, you'd have to reproduce its spec on simpler silicon. The market wasn't interested in an 8 MHz 68000 in 1991/92: the PC had stolen all of the Amiga's thunder at commodity prices. The Amiga's niche silicon was just too ...


13

'Jeff Porter realized it would not be possible to significantly cost reduce the Amiga 500 to get it into the $250 retail price range'. They could have cost-reduced the A500 - perhaps even to $250 retail - but they would have had to make some compromises that (thankfully) they weren't willing to do. There were a lot of chips on the board. They needed ...


12

In a double-sided drive, there are two heads which are necessarily closely spaced, facing both sides of the thin magnetic-coated disc once it is inserted. The concern would have been that the heads could collide with each other if the drive was jarred. However, the drive designs I'm familiar with have a mechanism to hold the heads and drive spindle away ...


11

In general: memory needs stable address for a while. So the real access time is "time to address available + memory access time". If you use the CPU with a full address bus (Z80, 6502, ...), it can expose the whole address in one cycle, wait only "memory access time" interval, and you can read. On the other hand, let's take the 8085 CPU with a multiplexed ...


10

According to EBU R95, the title-safe area for 576i format (corresponding to PAL SDTV) is 258 lines tall in each field. This is just large enough to accommodate the 256 lines per field that the BBC Micro uses. This is probably not a coincidence, as the BBC Micro was in part designed so that the BBC itself could use the micro for generating titles and ...


9

But as I understand it, the then current 16kbit RAM chips actually required three different voltages: -5, +5, +12. Right, and the 'missing' voltages (-5V,+12V) get generated from the +9V source via a discrete DC-DC converter - that's all the little pieces on the second board. The difference between the RAM-Packs for ZX80/81 is all in the colour :) In fact, ...


8

The 65816 does the same thing; the most-significant 8 address bits are multiplexed onto the data bus pins during the Phi1 half of each clock cycle, and it reverts to being a data bus during the Phi2 half. The WDC datasheet illustrates a simple external logic circuit which latches the address bits and isolates the data bus pins from a device responding ...


8

TL;DR Did historical sprite systems provide unrestricted positioning and overlap It wasn't unlimited and unrestricted, but limited by chip resources or memory bandwidth - or in case of inbetween systems by both. because the designers believed this was very valuable in reducing game development cost? No. Keep in mind, they often crippled machines ...


8

Did one or the other have a pass-through for the expansion bus? There was no other peripheral (by Sinclair) for the ZX80 than the 1..3 KiB static RAM or the later 16 KiB DRAM version (*1). Most (*2) third party vendors made theirs offering a pass-through. Did one or the other have a pass-through for the expansion bus? When the ZX-Printer came for the ...


7

The usual answer to questions on early Sinclair machines is "Because it was cheaper that way." In this case, a 40-column screen, implied by 4MHz, would have increased the screen buffer size to at least 960 bytes, leaving only 64 bytes for BASIC code. Increasing the RAM size would have raised the price point, and "Under £100" was crucial to the early ...


6

The 8088 (and Z80 and 1802 and many other older CPU designs, including minicomputers and mainframes) required multiple clock cycles to run each machine code instruction. This was due to being implemented by internal microcoding, limited shared resources, or non-pipelined state machines, due to much lower transistor counts than todays processors. As long ...


6

Answering your rephrased question: The master clock could have run at 8 MHz, but that would have meant pixels being narrower than if generated by a 6.5 MHz clock. As the computer was built with a horizontal resolution of 256 pixels in mind (the time spent by 32 NOP instructions), the border area would have been enlarged, resulting in a smaller display. ...


6

In this document there is a mentioning of half a million dollars spent for designing the Z80.


5

The VIC-20 video interface (MOSTEK 6560 / 6561) had 3 outputs of interest here: Audio Video which was output as Y/C (S video); the S-video was converted to composite video as shown below: Source These 2 signals (composite video and audio) ended up on the 5 contact DIN connector. To connect to a TV, an external RF modulator was required and this was ...


5

RF encoding introduces significant noise to the video signal because it moves the signal up to an area of the spectrum that is much more susceptible to interference. A monochrome 240p video signal of the best quality will be defined by its dot clock frequency at ~14 MHz. This would create a good quality 80 column text image as seen on machines like IBM CGA ...


5

Why was Apple unable to comply with the limits when e.g. Atari managed it? Purely through engineering that Apple was unwilling to carry out. I have added a relatively detailed explanation of this to the Atari 8-bit article on the wikipedia. The long-and-short is that it wasn't the slots themselves that were the problem, but providing some sort of ...


5

if the programmer can guarantee that sprites will never overlap each other, and that they will be presented in numerically increasing order on each scan line. A hardware designer's response would be "programmers can't actually guarantee that." And they'd be right. The hardware would have to be designed to do something sensible if those rules were broken. ...


4

Supposing you have a fixed pixel output clock then the bottlenecks are: shifters, since you need to be sure you may need to sample any sprite at the current location; and either: bandwidth to fill those shifters, if you're a TMS descendant (which includes all 2d Sega consoles) and are fetching sprite contents from regular video RAM; or storage for what ...


4

You're in luck — I believe that tape reading and writing is already CPU speed independent. For reading, the 1-bit tape input is provided as the CA1 input to the keyboard VIA. The OS sets it up to generate an interrupt on its rising edge. At each interrupt it interrogates the VIA's timer to determine how long the most recent wave was. So as long as you don'...


4

This doesn't answer the question regarding circuit implementation, but the Wikipedia articles on NTSC and Broadcast televistion systems have some relevant background, at least for NTSC used in the US. It's possible that PAL differed or that there were variations to NTSC for other countries, so if one of those is your primary interest please update your ...


4

The BBC Micro simply led the sound output to an internal speaker. On the circuit diagram, the output of the 76489 sound chip and the optional TMS5220 speech synthesiser are mixed, low-pass filtered and amplified through a series of ordinary op-amp based circuits. There is also a take-off, just before the final amplifier, to an external output jack.


4

FLAG is/was used to control behaviour of the language processor (compiler). It is to be placed between MODULE and TITLE. Its argument would be processed as if it was part of the command line. Command line settings would (usually) overwrite the same settings in FLAG. It was deprecated due its usage being implementation dependant. It is still processed by ...


3

The speed rating of asynchronous DRAM devices is usually (as in this case), the "RAS access time" or tRAC. This is the minimum guaranteed time for data to appear at the output after /RAS is provided with the row address, noting that this process also requires /CAS to be provided with the column address at some intermediate time. The speed at which you can ...


3

The Sinclair ZX Spectrum, for example, has the most simple analog circuit you could see in the early 80's home computers. The ZXS uses only one bit for a sound generator (and for tape output). In the Issue 2 schematics, you can see the whole sound circuit in the upper left corner: It's the speaker and two diodes. (Speaker is an inductive device, so you have ...


3

The only detail you’re missing is that A15 must be active in order for a processor M1 cycle to be treated as a video fetch. The full memory map is: 0000–3fff: ROM; 4000–7fff: RAM, no special handling; 8000–ffff: RAM, with M1 cycles subverted into video fetches. Both ROM and RAM just mirror within their areas. That’s 16kb granularity, so the top two ...


3

Not answering answering your question, but "black and white" TVs did produce a "better" picture as they didn't have the wire mask behind the glass, which was often very coarse on early TVs and even monitors. You could sometimes/often get a better grey image on a B&W TV than colour on a monitor.


3

I can answer half of that only: the BBC did often run into trouble, to the point that they cite the *TV command as early as Page 17 of the user guide: If the picture on your television screen is either too far up or too far down the screen, you can move the whole display with the command *TV. *TV 255 will move down one line *TV 254 will move ...


3

The other posts about hardware and gate costs better answer your question, but I'll add this as a counterpoint: A situation where a game programmer decided not to take advantage of hardware collision detection (in this case, for the Atari 8-bit port of Super Pac-Man): On the 400/800 I noticed that people knee-jerked toward using the player-missile ...


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