This is what the reset vector looks like in the ROM file:Īnd now we write a very short counter for the 5A22. If, for some reason, we want to start writing instructions at address 0x000004 in our file, the reset vector has to have to value 0x8004. So, instead of writing 0x0000, we have to write 0x8000, the the processor will start reading instructions from the beginning of our file. The reset vector is stored at address 0x007FFC and 0x007FFD, but remember, this address corresponds to 0x00FFFC and 0x00FFFD for the SNES! In our ROM file, we look for address 0x007FFC and 0x007FFD and write the values for the first address of instructions IN SNES ADDRESS SPACE. The interrupt vector table is located at the end of the first ROM bank, so around address 0x007FFF. The are several other interrupt vectors and they all have a certain location in memory, but at this point we only need the reset vector. The reset vector tells the processor where it can find the first value for the program pointer. Then the 5A22 boots up, it starts looking for the first address to read instructions from.
#BSNES VS SNES9X COMPLICATED CHIPS SOFTWARE#
While the name of the ROM and certain region locks are not important, because they are only tested by software itself, it is very important to provide parts of the interrupt vector table. This portion of the ROM is also known as the SNES header.
#BSNES VS SNES9X COMPLICATED CHIPS CODE#
The file needs a block of specific information which includes the internal name of the ROM, the interrupt vectors (addresses to machine code within the ROM in 16 bit format), version, etc. One more important thing is that ROM is only visible from SNES addresses 0x008000 to 0x00FFFF, because the lower half is used for other things. This concept works for banks up to 0x1F, then things get a bit more complicated, but I wouldn't worry about it too much right now. So, since one bit (A15) was left out, the ROM gets mapped twice on the SNES and has its own address space. Now the first bank bit increments and the SNES is at address 0x010000. This means the ROM is now at address 0x007FFF. So the SNES counts the addresses from 0x000000 up to 0x007FFF, everything is cool, then it counts from 0x008000 up to 0x00FFFF and actually sees the same data. But if we take SNES signals A16 to A23 (the bank lines), we can see that A16 is actually connected to A15 on the ROM. So the ROM is repeated once in every bank and the upper banks repeat too. Since A23 is ommited, the ROM can only distinguish between adresses in the banks from 0x00 up to 0x7F (128 banks) and the banks from 0x80 to 0xFF refer to the same area. Since A15 is not connected, the adresses from 0x008000 up to 0x00FFFF refer to the same area. So for addresses from 0x000000 up to 0x007FFF everything works "normal". It is produced by connecting the address lines like this: LoROM causes ROM chunks in SNES to be max 32KB wide ($8000 bytes), and are typically fetched from the $8000-FFFF region. There are two typical types of ROM for the SNES called LoROM and HiROM. There are ways to address much more space on the SNES, but for now, we don't care about this. So the ROM is located on 64 banks of 32768 bytes each.
To talk to the ROM, we have to use the correct address space.Īs you can see in the Memory Map of the SNES ( ), ROM is located at Banks 0x00-0x3F on addresses from 0x8000 until 0xFFFF. everything is connected to the address bus and seperated into address spaces called a "memory map". It has a 24-bit address bus and an 8-bit data bus, and everything is connected to these busses. Unlike processors of today, the Ricoh 5A22 (65816 core) does not have built-in memory. To write our first program we have to understand at least a little bit about memory mapping in the SNES. bsnes+ also emulates SPC-700 and other chips in the SNES. The bsnes+ emulator simply takes machine code from a binary file and feeds it to a virtual 65816 processor in a SNES machine. Emulators like 9snesx will not work because they ommit certain hardware "features" (and bugs). We'll have to use bsnes+ because it's emulating hardware and it's not focused on playing SNES games. bsnes+: A SNES emulator with debugging tools.There are many tutorials out there for SNES code in assembler language, but we're going to do things a little differently today: We're going to write bare-metal machine code right away! So, as promised, we're going to write our first SNES program today.
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