Because the PPU can only fetch an attribute byte every 8 cycles, each sequential string of 8 pixels is forced to have the same palette attribute. The data fetched from these accesses is placed into internal latches, and then fed to the appropriate shift registers when it's time to do so every 8 cycles. Each memory access takes 2 PPU cycles to complete, and 4 must be performed per tile. The data for each tile is fetched during this phase. The value on the PPU address bus during this cycle appears to be the same CHR address that is later used to fetch the low background tile byte starting at dot 5 possibly calculated during the two unused NT fetches at the end of the previous scanline. Eu4 launcher exeĭuring these scanlines, the PPU is busy fetching data, so the program should not access PPU memory during this time, unless rendering is turned off. This includes the rendering of both the background and the sprites. These are the visible scanlines, which contain the graphics to be displayed on the screen. During pixels through of this scanline, the vertical scroll bits are reloaded if rendering is enabled. However, this behavior can be bypassed by keeping rendering disabled until after this scanline has passed, which results in an image that looks more like a traditionally interlaced picture. NES Programming #26 - We scrollin', I'm loadin' This is done to compensate for some shortcomings with the way the PPU physically outputs its video signal, the end result being a crisper image when the screen isn't scrolling. For even frames, the last cycle occurs normally. This scanline varies in length, depending on whether an even or an odd frame is being rendered.įor odd frames, the cycle at the end of the scanline is skipped this is done internally by jumping directly fromto 0,0replacing the idle tick at the beginning of the first visible scanline with the last tick of the last dummy nametable fetch. Although no pixels are rendered for this scanline, the PPU still makes the same memory accesses it would for a regular scanline. This is a dummy scanline, whose sole purpose is to fill the shift registers with the data for the first two tiles of the next scanline. The current pixel for each "active" sprite is checked from highest to lowest priorityand the first non-transparent pixel moves on to a multiplexer, where it joins the BG pixel.Įach scanline lasts for PPU clock cycles The line numbers given here correspond to how the internal PPU frame counters count lines. This output accompanies the data in the sprite's latch, to form a pixel. If the counter is zero, the sprite becomes "active", and the respective pair of shift registers for the sprite is shifted once every cycle. For each sprite, if the counter is still nonzero, nothing else happens. Every cycle, the 8 x-position counters for the sprites are decremented by one.
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Afterwards, the shift registers are shifted once, to the data for the next pixel. Every cycle, a bit is fetched from the 4 background shift registers in order to create a pixel on screen.