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Questions tagged [memory-layout]

For questions regarding the layout or mapping of memory in a retrocomputer.

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How does the 8086 jump instruction work when the target address overflows?

For educational purposes, I'm developing my own software emulator for the 8086 microprocessor with VGA/MCGA support. Although it's far from complete, it's advanced enough to start using a BIOS (though ...
ExecAssa's user avatar
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15 votes
4 answers
4k views

Can DOS make use of more than 640 KB of conventional memory on 80186?

Can I have more than 640 KB conventional RAM under some DOS? I'm looking at an 80186 PC that has RAM at address A0000. Video RAM starts at B0000 as expected for a Hercules; A0000 is just more main ...
Joshua's user avatar
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14 votes
3 answers
3k views

Why does PC video memory base address change depending on video mode?

It is rather well-known that, with VGA-compatible PC video adapters, in black-and-white text modes video memory is available at linear address 0xB0000, in colour text mode at address 0xB8000, while in ...
user3840170's user avatar
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8 votes
1 answer
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C64 char screen plot routine - not clear about reason for logical OR

I'm trying out some 6502 assembly language, specifically using Kick Assembler. I have the following routine that just sticks a character on the text screen at a specified x and y, it's taken from the ...
Alan B's user avatar
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13 votes
1 answer
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What problem does LOADFIX solve?

From MS-DOS help for the LOADFIX command: Some programs will display the "Packed file corrupt" message when all or a portion of the program has been loaded in the first 64K of conventional ...
user10191234's user avatar
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10 votes
2 answers
2k views

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 ...
pts's user avatar
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11 votes
1 answer
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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 ...
pts's user avatar
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22 votes
4 answers
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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 ...
Schmuddi's user avatar
  • 935
19 votes
6 answers
5k views

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 ...
Flux's user avatar
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11 votes
2 answers
687 views

Why did the COMX-35 video memory seem unreliable?

In my early days, many many years ago, one of the machines I managed to get my hands on was the COMX-35, a funky little machine based on the 1802 CPU. I remember trying to develop some games for it ...
paxdiablo's user avatar
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23 votes
3 answers
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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 ...
paxdiablo's user avatar
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2 votes
1 answer
348 views

When was fixed page size, flexible assignment bank switching patented?

The 8-bit microprocessors invented in the seventies, had a 16-bit address space. It didn't take long for memory demand to exceed this, with the result that bank switching was a fact of life for the ...
rwallace's user avatar
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22 votes
5 answers
4k views

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 ...
Shades's user avatar
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18 votes
3 answers
6k views

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 ...
rwallace's user avatar
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6 votes
3 answers
1k views

How did Apple IIe software handle 64K bank switching?

The early microcomputers were well served by 8-bit CPUs with 16-bit address bus width, but later generations of these machines tended to run out of address space, and had to resort to some form of ...
rwallace's user avatar
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9 votes
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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 ...
Anthony's user avatar
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19 votes
3 answers
5k views

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 ...
anmomu92's user avatar
  • 193
5 votes
1 answer
689 views

How to assign code to different PRG-ROM banks on an NES ROM using ca65/ld65

A similar question was asked here but I read it and didn't really understand the solution. What I'm trying to do here is make an NES ROM that uses Mapper 24, aka Konami VRC6 Version 1 (the one used by ...
puppydrum64's user avatar
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16 votes
2 answers
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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 ...
user3840170's user avatar
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5 votes
1 answer
482 views

How did games written for tape-based BBC micro, get officially and unofficially ported to disk, bearing in mind extra workspace needed by DFS ROM?

If I recall, PAGE = &E00 for a BBC Model B system with cassette tape based storage. On installation of Acorn DFS ROM (with 8271 Floppy disc controller), the ROM allocated more memory, increasing ...
therobyouknow's user avatar
5 votes
2 answers
674 views

What is the reason of the particular range of the last 4K block of memory selection in Apple II

Following my previous question on Identifying the functionality of the memory select in Apple II and now that I know that the memory select device is simply an interconnection block. Searching for ...
Paul Ghobril's user avatar
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6 votes
1 answer
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Bank switching and memory perspectives (C64)

I've been reading up on bank switching on Wiki and in particular here which contains the below nice image and have a few questions: Am I right in thinking that LOAD loads into RAM (dark grey in the ...
user avatar
4 votes
2 answers
1k views

Transforming a memory dump into something loadable

As a follow-up to this question, I now have: A memory dump of the unpacked application The start address of the application I can verify that the dump is correct by starting a new instance of Vice, ...
Cactus's user avatar
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8 votes
2 answers
989 views

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....
knol's user avatar
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9 votes
2 answers
309 views

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 ...
Rhialto supports Monica's user avatar
26 votes
8 answers
13k views

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 ...
rwallace's user avatar
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9 votes
1 answer
2k views

What is the memory layout in MS-DOS

I know that when an .COM file is loaded, DOS loads its contents into memory, sets the segment registers (CS, DS and SS) to point to the 64KB segment and then performs a jmp to the starting address. ...
DarkAtom's user avatar
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7 votes
2 answers
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Details of ZS Scorpion port 0x1ffd

There's this page about the ZS Scorpion, and I'm reading about the way bankswitching works. Port 0x7ffd is the same as on western ZX Spectrums, but to double the amount of RAM to 256k and also the ...
Omar and Lorraine's user avatar
11 votes
3 answers
2k views

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 ...
DarkAtom's user avatar
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16 votes
2 answers
4k views

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 $...
Bart Friederichs's user avatar
4 votes
7 answers
560 views

Static memory partitioning

Does anyone know of any operating system that used static memory partitioning: contiguous physical memory allocation with one process per partition, one partition per process, partitions generated at ...
DYZ's user avatar
  • 275
9 votes
2 answers
2k views

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 ...
CJ Dennis's user avatar
  • 2,517
12 votes
1 answer
2k views

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, ...
rwallace's user avatar
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16 votes
1 answer
2k views

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 ...
natevw's user avatar
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6 votes
2 answers
2k views

How did the BBC sideways ROM software for the AMX mouse process the user port input data to determine x and y movement?

This mouse plugged into the user port on the underside of the BBC micro models. The mouse came with software in the form of a sideways ROM which provided APIs, CLI commands for own programs as well as ...
therobyouknow's user avatar
22 votes
1 answer
3k views

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 ...
user's user avatar
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9 votes
10 answers
1k views

Were there any games/software that used memory beyond what was advertised available to BASIC on the machine?

Were there any games/software that used memory beyond what was advertised as available to BASIC on the machine ? On home / personal computers any time up to 1984 . Without needing to plug in any ...
questiontype's user avatar
  • 1,061
3 votes
2 answers
213 views

virtual addressing in device drivers

Sun's SBus is particular for having virtual addressing and address translation even for device drivers. Any other buss design which had this peculiarity ? Check out Ben Catanzaro's book The SPARC ...
Stefan Skoglund's user avatar
14 votes
1 answer
2k views

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....
Richard Downer's user avatar
10 votes
2 answers
1k views

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 ...
RetroSpark's user avatar
10 votes
3 answers
5k views

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 ...
Jasmine's user avatar
  • 203
7 votes
4 answers
2k views

How to use HGR2 (or HGR) on an Apple II

I am a big fan of the TV show lost, and I was trying to make the Dharma Initiative logo. But at certain point I couldn't add new lines of code and the final part of the logo was cut off. I tried to ...
Arthur Oribe's user avatar
10 votes
2 answers
1k views

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 $...
secondperson's user avatar
32 votes
1 answer
3k views

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)...
smitelli's user avatar
  • 1,779
7 votes
5 answers
921 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 ...
user14821's user avatar
29 votes
3 answers
7k views

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 ...
Brian H's user avatar
  • 60.9k
2 votes
3 answers
255 views

Paging vs. segments in the Z8000

Having post-dated either, I came to understand the difference between paging and segmentation to be that the former refers to fixed blocks while the later can point anywhere. To do so, the segments ...
Maury Markowitz's user avatar
4 votes
2 answers
316 views

ISA Extension cards using more than 64kB

Related to my previous question around VGA framebuffer being limited to 64kB I started to wonder if there was any extension cards (or similar) using more than 64kB of address space in linear fashion, ...
tuomas's user avatar
  • 2,793
10 votes
3 answers
2k views

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 ...
tuomas's user avatar
  • 2,793
9 votes
5 answers
3k views

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 ...
MastErAldo's user avatar