SNES games are intended to keep running until the user turns the system off or presses reset. Unlike many computer programs, SNES programs aren't really designed to end. Lda #%00001111 End VBlank, setting brightness to 15 (100%).Įnding the program We end the VBlank by turning the screen back on and setting the brightness to 15, again using the Screen Display Register ($2100): Lda #%00000000 Load the high byte of the green color. Lda #%11100000 Load the low byte of the green color. We set the background color of the screen using the Color Data Register ($2122): The color that we will use is 00000000 11100000 (0 blue, 7 green, 0 red) - a dark green. The SNES stores 16-bit colors in the following format: Lda #%10000000 Force VBlank by turning off the screen. Here, we force VBlank by turning off the screen, using the following code: Turn off the screen by setting the eighth bit of the Screen Display Register ($2100).Wait for a VBlank non-maskable interrupt ( NMI).There are two ways to ensure that your code executes during a VBlank: VBlank occurs once per field, or about 60 times per second on NTSC machines and 50 times per second on PAL machines. Since VBlank is much longer than HBlank and covers the whole screen, most updates should be done during VBlank. Therefore, you want to make all your modifications to the screen during HBlank or VBlank. If you update the screen while the electron beam is turned on, the results displayed to the user are unpredictable they may shear or have other artifacts. Likewise, between fields, when the electron beam is being moved from the bottom of the screen to the top, the electron beam is turned off this is called vertical blank or VBlank. Certain special effects - like the perspective effects of the Final Fantasy VI airship - can be performed on the SNES by changing graphics settings during HBlank. Thus, an NTSC television displays about 60 fields per second, while a PAL television displays 50 fields per second.īetween lines, when the electron beam is being moved from the right end of one scanline to the left beginning of the next scanline, the electron beam is turned off this is called the horizontal blank or HBlank. and Japan) displays roughly 30 frames per second, while a PAL television (as used everywhere else) displays 25 frames per second. Additionally, television images are interlaced, meaning that the television alternates displaying a screenful of all the even-numbered scanlines with a screenful of all the odd-numbered scanlines each screen of even- or odd-numbered scanlines is called a field. The SNES refreshes its screen to match the signal output to the television, so knowing how the television updates its image can help you understand how certain special effects are performed in the SNES.Ī television displays an image on the screen using an electron beam, which it sweeps across the screen from left to right, one horizontal line at a time from top to bottom. We do this by setting the appropriate bit in the CPU status register: Setting the background color įirst, we set the accumulator to 8 bits, to modify single bytes of RAM. Here, we use the initialization macro Snes_Init from the file " Snes_Init.asm" from the SNES Devkit, which does this for us: To reset the SNES, we set a number of hardware registers to zero and zero all the bytes in WRAM. Resetting the SNES, however, may be necessary for the demo to run on actual SNES hardware. Since emulators probably start in a known state, it may be unnecessary to reset the state for an emulated ROM. Most demos reset the SNES to a known state. In the header file, the label " Start" is declared as the reset vector, so that is where the program shall begin executing: While not necessary to get this project to work correctly, it should be noted that in order for the VBlank subroutine to actually be called, you would need to enable NMI interrupts as below. You can find the ASM-code for the Snes_Init.asm here on Wikibooks, and you can find the header here. This part can be skipped by putting the header and init code in your main file. VBlank: Needed to satisfy interrupt definition in "header.inc" If you decide to put the init routine and the header in separate files, it will be necessary to tell the assembler to include them. The header contains assembler directives for WLA which do things like define the ROM's name, size, vectors, etc. The initialization routine is self-explanatory it's a piece of code that clears/resets system values and sets the SNES up for use. An initialization routine and a header.A SNES emulator, such as bsnes, Zsnes or Geiger's debugging version of Snes9x, for windows.2.4.2 Step 2: Setting the Background Color.
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