Questions tagged [memory-layout]

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

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4 votes
2 answers
308 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, ...
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 ...
15 votes
4 answers
3k 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 ...
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 ...
23 votes
3 answers
4k views

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 ...
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)...
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 ...
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 ...
12 votes
1 answer
2k views

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 ...
9 votes
2 answers
609 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 ...
8 votes
1 answer
1k views

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 ...
13 votes
1 answer
1k views

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 ...
7 votes
2 answers
411 views

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 ...
32 votes
6 answers
4k views

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 ...
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 ...
11 votes
1 answer
573 views

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 ...
7 votes
2 answers
2k views

IBM PC memory map - why RAM at the bottom?

The 8088 provided an address space of one megabyte. The IBM PC allocated that address space as 640K RAM (not that the 5150 could physically take that much, but the address space was allocated) ...
16 votes
2 answers
1k views

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 ...
22 votes
4 answers
3k views

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 ...
20 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 ...
55 votes
8 answers
11k views

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. ...
9 votes
2 answers
3k views

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 ...
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. ...
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 ...
2 votes
1 answer
344 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 ...
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 ...
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 ...
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 ...
11 votes
2 answers
1k views

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 ...
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 ...
11 votes
3 answers
2k views

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 ...
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 ...
5 votes
1 answer
644 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 ...
8 votes
2 answers
972 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....
5 votes
1 answer
452 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 ...
14 votes
5 answers
4k views

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 ...
5 votes
2 answers
667 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 ...
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 $...
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 ...
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....
6 votes
1 answer
1k views

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 ...
20 votes
2 answers
3k views

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 ...
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, ...
9 votes
2 answers
302 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 ...
10 votes
1 answer
964 views

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, ...
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 ...
62 votes
3 answers
17k views

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 ...
12 votes
2 answers
857 views

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 ...
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 ...
29 votes
2 answers
4k views

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 ...