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Questions tagged [memory-layout]

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

12
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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 ...
29
votes
7answers
7k 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
votes
1answer
318 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 ...
4
votes
1answer
364 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
votes
2answers
949 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 ...
8
votes
2answers
357 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
votes
2answers
182 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
votes
1answer
197 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
votes
1answer
438 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 ...
16
votes
1answer
892 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 ...
9
votes
4answers
1k 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 ...
9
votes
3answers
803 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 $...
10
votes
1answer
1k 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-...
12
votes
2answers
1k 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
votes
1answer
544 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 ...
11
votes
1answer
346 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, ...
13
votes
2answers
523 views

What is this code on the zero page that was put there by the BASIC ROM?

Zero-page memory maps of the PET that I've found claim that the zero page address range$00C2..$00D9 are used for static data, e.g. http://www.classiccmp.org/dunfield/pet/petmem.txt says: RIDATA 00C2 ...