flipacholas
commented
3 years ago Thanks for all these corrections! I'm going over them just now and I'm only having problems with the backwards compatibility one. I'm reading some disassemblies and docs of the GBA BIOS and I can't find much info about the GBA boot process (specifically, when and how it switches to GBC mode).
It turns out that the GBA BIOS does see the cartridge slot switch get pressed, and it reacts by switching the system into GBC mode.
Could you tell me where that info was found? So far, thanks to this project I can back up how the GBA switches to GBC mode in software, but tcrf lists the GBC boot routine as unused.
Edit: I've just realised you are the author of that program! The statement I'm trying to back up is if during boot, in order to switch into GBC mode with a GBC game inserted, the GBA boot code checks the state of the switch and just changes DISPCNT. Leaving the ARM7 core still working (hence why the display can still be tweaked?). Then I'm guessing the GBC CPU is in charge?
Dwedit
selb
AntonioND
This is regarding the article https://www.copetti.org/writings/consoles/game-boy-advance/
Diagram
In the Diagram, it shows the GBC cartridge having a mapper, and the GBA cartridge not having a mapper.
This is true for the vast majority of games, but a few games on the GBA were 64MB in size (such as GBA Video cartridges), and needed a mapper to address the whole 64MB cartridge. Flash cartridges also used mappers.
Memory locations
4 locations are listed there. IWRAM, EWRAM, PAK ROM, and CART RAM.
IWRAM is the fastest memory available on the system. It has a full 32-bit bus, and executes 32-bit ARM instructions at full speed, and reads or writes 32-bit data words at full speed. But it's only 32KB, so "Big Chunks" doesn't sound like the correct term to describe it. I understand that "big chunks" means 4-byte words here, but the term seems a little misleading. You can't hold that much data into the tiny 32K of IWRAM.
Also, Video RAM is missing here. Unlike the NES, SNES, or GB/GBC, the CPU has full access to video memory whenever it wants it, it's not just for the PPU. While it is on a 16-bit bus, it still has an extremely fast connection to the CPU. It only takes 2 cycles to execute a 32-bit ARM instruction from Video RAM, and takes 1 cycle to Read/Write a 16-bit word, or 2 cycles to Read/Write a 32-bit word. This means that Video RAM can be used as "More IWRAM" if you really need the space for running more fast code, or storing more fast data.
The speed comparison is like this:
The bus size is incorrect here. It's an 8-bit bus, not a 16-bit bus. Furthermore, it's the only bus where you can't read or write more than 8 bits at a time. Attempting to read more than 8 bits will only give useful data in the low 8 bits of the word read back.
"Critical Sections"
The term "critical section" refers to concurrent programming, or multiple threads. When code is placed in a critical section, it can only be executed by one thread at a time, other threads need to wait to get in.
Perhaps change "critical sections" into "High-speed 32-bit code in fast memory".
How do they maintain compatibility?
It turns out that the GBA BIOS does see the cartridge slot switch get pressed, and it reacts by switching the system into GBC mode. The GB Micro can also be swtiched into GBC mode by software, but it is unable to read GBC cartridges.
Constructing the frame
The caption on the background says;
The "Affine" background layers refer to the "Mode-7" like scaling and rotation effects used by games like Mario Kart or F-Zero. The three background layers seen here are plain background layers, not affine-transformed layers.
"Raster effects" (scrolling changes performed at each scanline) can be used to perform some distortions that keep the scanlines whole. This includes effects such as horizontal skewing, simulated multi-plane parallax, wavy scrolling, vertical scaling, or other vertical wavy effects. Raster effects can be performed on plain background layers, the actual Affine layers aren't needed for those.
It's not quite three 32KB independent blocks.
When you set up a background layer, you can pick one of Four different possibilities for which graphics to use. Each possibility is 32KB large, but they can start at any multiple of 16K. It could be 0K-32K, 16K-48K, 32K-64K, or 56K-64K (can't go past 64K for the background tiles, remaining 32K is for sprites only)
You also pick where the Tile Map goes. You can see some of the junk graphics sitting on "Block 2" (background 32K-64K) that look like vertical lines, those are the tile maps.
Only "Block 3" (Sprite 64K-96K) is reserved for a specific use in that video mode, and that is for sprites.
Not quite true anymore, the CPU has full access to Video RAM whenever it wants it, so there is no need to wait for Vblank time to perform DMA like on the other consoles. The only possible consequence is screen tearing due to the graphics changing the middle of the frame.
Cartridge space
The address space is 64K large, but Retail GBA cartridges did not exceed 32KB of SRAM. Flash cartridges did offer 64K of SRAM though.
Flashcards
The earlier flash cartridges did not have any memory card slots. Instead, the entire cartridge was NOR Flash memory, plus battery backed SRAM for saves. You booted a small program over the link cable which would rewrite the cartridge, using data received over the link cable.