Questions tagged [memory-layout]
For questions regarding the layout or mapping of memory in a retrocomputer.
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Who set the 640K limit?
We all know that "640K should be enough for everyone". But who actually set this limit? The quote is often attributed to Bill Gates, but it doesn't seem like a decision for an Operating System vendor ...
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Why didn't the 8086 use linear addressing?
The 8086 used a segmented memory architecture where the linear address was computed from a 16-bit segment number and a 16-bit offset. This greatly complicated things from a programming perspective. ...
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Did DOS zero out the BSS area when it loaded a program?
As an example, say we have a DOS MZ EXE file that's around 20 KiB in size. The EXE header contains the value 0x1400 at offset 0x0A indicating that the program is requesting 5,120 paragraphs (or 80 KiB)...
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Why is the Amiga ROM at a high memory location, and RAM in low memory?
When a 68000 CPU powers up, it reads a few words at memory location zero to get the initial stack pointer and program counter. That suggests to me that a computer system designer would put the system ...
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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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How did an IBM 5150 with 16KB RAM work?
I keep reading that when it was first released, there was a variant of the IBM PC model 5150 that had only 16KB of RAM installed. From a hardware perspective, this would clearly work - the 5150 ...
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Did any computer use a 7-bit byte?
In an answer to Why did IBM System 360 have byte addressable RAM I wrote regarding the choice of byte size:
7 bits would be a perfect match for ASCII, but engineers would instinctively recoil from ...
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What were the actual memory model definitions in MS-DOS?
I've heard the phrase "memory model" used in relation to MS-DOS programming (and early Windows), with terms such as "small" and "compact".
But what were the actual ...
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Was it possible to cause persistent changes to a mid-1980s IBM-PC using POKE in GWBASIC?
TL;DR: Using the DEF SEG and POKE commands in GWBASIC, was there any way to make changes to an IBM-PC compatible computer that would (a) persist even after a reboot and (b) cause an increase in crash ...
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How is memory allocated in Super Mario World?
Super Mario World (SMW) is known to have several bugs relating to re-use of memory for multiple purposes. For example some memory items are used for more than one type of item, and the designers are ...
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How did old computers address far more than 64K of memory despite only having a 16 bit address bus?
I have an old Sharp PC-G830 pocket computer from the '80s that has 32K of RAM and 256K of ROM. I also have a simple single board computer I built with 128K of RAM and a few megabytes of ROM from a ...
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Where is DOS stored in memory when a program starts?
When the execution of a COM program begins, DOS jumps to address 100h. But at what address is DOS stored in RAM while the COM program is executing? Is DOS stored in conventional memory? If so, isn't ...
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Was 1991's Hellcats the first instance of incremental screen updates?
In case you have never seen it, 1991's Hellcats was a seminal release on the Mac. It ran at full 8-bit color and could, on a newish machine, drive three 1024x768 screens at the same time. Nothing on ...
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How to check the C64 graphics mode used by Maniac Mansion
I have been investigating the MS-DOS port of the C64's Maniac Mansion game. The original MS-DOS port used the C64-specific character map to draw the various backgrounds in the game. I noticed that ...
user4411
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What is the purpose of mirrored memory regions in NES's CPU memory map? [duplicate]
[Please see answers to this related question as well]
I've started reading the "official" NES Documentation and in page ten, it says that "memory locations $0000-$07FF are mirrored ...
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Why did the TI-99/4 have two databuses?
Wikipedia says on Russian page:
Однако к 16-разрядной шине были подключены только 256 байт статической памяти и системное ПЗУ. Остальная память (ОЗУ) и периферийные устройства были 8-разрядными и ...
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Did anyone ever put half a megabyte of memory in an Altair?
The Altair 8800 typically, at least in the early years after its release in 1975, operated with no more than a few kilobytes of memory, for the excellent and sufficient reasons that memory was ...
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How did the Sideways address space work?
I was researching the BBC Micro when I found the Sideways address space: a 16KiB memory space allowing access to up to 16 16KiB external blocks of ROM (or RAM).
But 16 blocks of 16KiB is 256KiB, ...
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How was a line of Commodore 64 BASIC code stored in its memory?
I am reading up on old C64 stuff and I am using the Vice emulator to play with it. Currently I am studying about how the C64 stored BASIC programs in RAM.
This is quite clear (it starts on address $...
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What are the "ports" used via IN/OUT, vs. the PEEK/POKE address space?
This is something of a followup to How much control of TRS-80 Model III disk drives was possible from its Cassette (ROM) BASIC? but on a more general topic:
I know that PEEK and POKE enable direct ...
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What is this code on the zero page that was put there by the BASIC ROM?
(question copied from SO (from the days before RC.SE existed): http://stackoverflow.com/q/31877835/477476)
Zero-page memory maps of the PET that I've found claim that the zero page address range$00C2....
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How much of the Program Segment Prefix area can be reused by programs with impunity?
I am writing a tiny TSR program, and I want it to take as little memory while installed as possible. The memory footprint of every loaded DOS process, including a TSR, includes a data structure known ...
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What did POKE 842,13 on Atari do? (Jamming the Return key)
In quite a few programs on Atari, I found the self-programming trick:
10 GRAPHICS 0 : REM clear the screen, enter standard text mode
20 POSITION 2,4 : REM place cursor 4 lines from ...
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Mapping more than 64kb of address space
I'm planning out a 6502 homebrew build but seem to be stuck in the issue of buying parts. My plan is to extend the addressing capabilities of the 6502 by giving myself 64kb of RAM, and another 64kb of ...
user7013
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How does the Commodore 64 address more than 64 kilobytes of memory?
So, the Commodore 64 has two special registers in locations $00 and $01. By writing to these registers, you can somehow turn on and off the ROMs and other things. The thing that I am not understanding ...
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What manages Upper Memory Blocks (UMBs) in MS-DOS?
I had always assumed that EMM386.EXE was responsible for managing the Upper Memory Blocks - UMBs - the memory space between 640KiB and 1MiB in real mode x86. For example, on MS-DOS 6.22, help emm386....
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What is this unused memory range in the Game Boy's memory map?
Here is a basic memory map of the Game Boy's 16bit address bus:
0000-3FFF 16KB ROM Bank 00 (in cartridge, fixed at bank 00)
4000-7FFF 16KB ROM Bank 01..NN (in cartridge, switchable bank number)
8000-...
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Is all 100% of a 64k Apple II memory usable?
Are there any areas of the 64k RAM which are permanently unusable by anything, whether by the built-in monitor ROM or user programs?
Looking at $C000-C0FF, this is the "softswitch" area, and as far ...
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Why did the Amstrad CPC use a nonlinear screen memory layout?
The screen memory layout on the Commodore 64 in bitmap mode was nonlinear - which incurred a penalty in development time, code size and speed for games using it - because when designing the VIC-II, ...
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Why did Amstrad choose such bank combinations for its all-RAM mode in +2A and +3 Spectrum computers?
The +2A and the +3 versions of the ZX Spectrum, made by Amstrad, had a new banking mode, called "special". This banking mode is entered by setting bit 0 of port 1FFD. When active, two bits from this ...
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Why are some soft switches on the Apple II only triggered with a write?
The Apple II uses memory mapped I/O and soft switches to do many things. One thing has confused me though: why are some soft switches only activated when written to?
For example, 80COLON ($C00D) and ...
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How can I access/use memory outside of the standard 1 MB address range of MS-DOS?
How do you access more memory (above the 1MB) in DOS if the 640KB of conventional memory are not enough?
I have read a lot about this, but I couldn't figure out how to do this in actual code. Is ...
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How does JSR actually work on the 65c816 CPU for the SNES (Super Nintendo)?
Take the following machine code for the 65c816 for the SNES (Super Nintendo):
00000000 ea ea 78 18 fb c2 18 a2 ff 1f 9a 20 fa 80 e2 20 |..x........ ... |
00000010 a9 80 8d 00 21 a9 e0 8d 22 21 ...
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Disable memory refresh on Z80
Is there a software method of disabling memory refresh on the Z80? Or would it be acceptable to create a hardware method of disabling the address bus during the refresh cycle? If I switch off the ...
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Why are the PPU registers on the NES mirrored?
[Please see answers to this related question as well]
The NES Picture Processing Unit has eight memory-mapped registers to the CPU in registers $2000 to $2007. The are incompletely decoded, so they ...
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Why VGA framebuffer was limited to 64kB window?
VGA framebuffer was fixed to 64kB at A0000h. Right after that there’s MDA/CGA framebuffer at B0000h. I am not sure, but I recall VGA did have to support CGA and its framebuffer, but was there any ...
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How to write Win16 program with only a single segment (combined code+data)?
I wrote a simple Win16 program in NASM assembly. It works on Windows 3.11. (Source code: https://github.com/pts/mininasm/blob/master/demo/hello/helljw16.nasm .)
When I tried to combine the code and ...
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Behaviour of $D000-$D3FF section on the C64 during bankswitching
My question is about bankswitching behaviour on the Commodore 64.
I'm writing a C64 game for a 16kb cartridge with EXROM and GAME pulled low, so the available bankswitching modes are (according to ...
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Can the Nintendo 64 run code directly from ROM?
I've read that the usual way to execute code on the N64 is to use DMA to copy it from ROM into RAM, and then run from RAM.
However, it seems that the contents of the ROM are directly visible to the ...
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Why is the Interrupt Enable Hardware Register in the HRAM area on the Gameboy?
It's something I've always been wondering:
Almost all of the hardware registers on the Gameboy are placed between $ff00 and $ff7f (with plenty of gaps and empty space), the HRAM is located between $...
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Amiga 500+ rev8A, 1 MB chip RAM and 0.5 MB slow RAM
I have an 500+ rev8A, I added the missing 0.5 MB RAM and the U32 to the motherboard. I also have 512 KB in the trapdoor. This results in 1.5 MB chip RAM; but I want to have 1 MB chip RAM and 0.5 MB ...
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C64 cartridge emulation with ATmega
As a personal project I had the idea to create a custom cartridge for my Commodore 64 and use an ATmega 1284p microcontroller to emulate eproms and/or custom chips.
Basically my idea is similar to ...
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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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Commodore Plus/4 60671 bytes free - how?
It is widely known that, while the Commodore 64 did indeed provide 64K of RAM for machine code programs, only 38K was usable from BASIC; this was because bank switching was needed to get at the rest, ...
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In the Amstrad CPC's Mode 0, what was the design rationale for interleaving the pixel bits?
The following tables are modified and corrected from Painting pixels: Introduction to video memory.
Mode 2 - 640x200 (half width pixels), 2 colours
memory bit
7
6
5
4
3
2
1
0
pixel
0
1
2
3
4
5
6
7
...
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Unused RAM Chips on x86 machines
On classic x86 machines the upper 384 KB of system memory contains video RAM and BIOS ROM Besides other things. Those areas overlay over conventional RAM, so that you can't use all upper memory, but ...
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Paradigm for (repeated) use of PDP-10 indirect bit
Answers to question PDP-10 effective address calculation explain how PDP-10 effective address calculation works, including potentially infinite indirections.
However the answers don't address how this ...
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How to use all memory on an IBM PC with 8086
I'm developing software for the IBM PC with an 8086 processor. I want my program to use all available memory.
I know that I can use DOS int 21h function AH=48h to allocate all available conventional ...
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How can I malloc() a block that's guaranteed to lie within a single DMA segment in Turbo C 2.01?
I'm following root42's videos about DOS programming using Turbo C 2.01. I've written my own Soundblaster 1.xx driver following the Creative Labs documentation, and I'm confused about memory allocation....
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How does Notepad store text files in memory?
Before I knew about line endings, I used to open small executables in Notepad and ruin them completely. However, some more complex executables (e.g. Paint Shop) were too big for Notepad, and I was ...
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