Questions tagged [memory-layout]

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

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26
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1answer
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)...
5
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5answers
340 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 ...
26
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3answers
5k 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 ...
2
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3answers
173 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 ...
2
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2answers
127 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, ...
6
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3answers
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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 ...
5
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3answers
428 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 ...
1
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2answers
421 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) ...
5
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1answer
310 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, ...
4
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1answer
220 views

How is the 320x200x16 graphics mode mapped in the IBM PCjr?

I've read that the PCjr supports a 320x200 pixel mode with 16 colors. But what I can't seem to find is how much memory that takes, how it's organized (chars?) or where it's located in memory. I'm ...
1
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1answer
171 views

Does the TurboGrafX16 store switches and/or flags in RAM?

Based the the specs I found for the TG16, it has 8 KB of RAM. I also found notes in the documentation of many opcodes that there is data stored in several "flags and switches" are these stored in RAM ...
3
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2answers
209 views

PDP-10 effective address calculation

How does effective address calculation work on the PDP-10? My understanding is that the instruction code contains an 18-bit address or offset, one bit for indirect, and a register code that's 4 bits. ...
16
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5answers
2k views

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 ...
45
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3answers
11k 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 ...
9
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1answer
390 views

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 ...
17
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4answers
3k views

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 ...
4
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2answers
332 views

How does the PMD 85 display colour or greyscales?

I stumbled across this computer in an article on hackaday, Home computers behind the Iron Curtain and since it has this bizarre resolution, I looked closer into it. Some claims from Wikipedia: ...
41
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7answers
8k 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. ...
3
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1answer
542 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 ...
6
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1answer
489 views

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 ...
9
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2answers
1k views

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 ...
9
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2answers
615 views

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 ...
7
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2answers
375 views

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 ...
3
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1answer
278 views

Can the BBC Master co-processor execute a program in the I/O processor's memory?

Whilst researching the 65C102's memory layout, it occurred to me that, since the I/O processor's memory is mapped to 0xFFFFxxxx, it might be possible to store code there and execute it by setting the ...
9
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1answer
733 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 ...
18
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1answer
1k 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 ...
10
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5answers
2k 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 ...
10
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3answers
1k views

Why are the PPU registers on the NES mirrored?

The NES Picture Processing Unit has eight memory-mapped registers to the CPU in registers $2000 to $2007. The are incompletely decoded, so they are mirrored every 8 bytes from register $2008 to $...
12
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1answer
2k views

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-...
22
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5answers
2k 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 ...
13
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2answers
2k views

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 ...
13
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1answer
674 views

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 ...
13
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1answer
557 views

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, ...
15
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2answers
760 views

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....