Differences

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--- graphics:memory [2023/08/22 23:50] – created clyde
+++ graphics:memory [2023/12/18 04:10] (current) – [Sprite RAM] clyde
@@ Line 1: / Line 1: @@
 ====== Graphics Memory ======
-The GameTank has two kinds of memory involved in graphics and video.
+The GameTank has two kinds of memory involved in graphics and video. Both are accessed in the range $4000-$7FFF when each is banked in by the system control registers.
+When DMA_ENABLE is unset then DMA_CPU_TO_VRAM determines whether the CPU accesses the Framebuffers (when flag is 1) or Sprite RAM (when flag is 0).
 ===== Framebuffers =====
@@ Line 15: / Line 17: @@
 The blitter may read anywhere within a 256x256 page as a contiguous region. When accessed by the CPU however, only one quadrant of the currently selected sprite page is mapped in at a time.
-The quadrant selected for CPU access is determined by the state of the blitter's sprite cursor. Whichever quadrant the sprite cursor was in at the end of the last blitter operation will be the quadrant accessed by the CPU in direct mode.
+The values of GX and GY last used by the Blitter will also influence which part of Sprite RAM the CPU can access in addition to the Banking Register. The video section of the memory map is only big enough for a 128x128 region. So the quadrant of Sprite RAM available to the CPU is determined by the most significant bit in the Sprite RAM coordinate counters. Typically these would be set before loading sprites by running a single-pixel blit operation copying from the target quadrant to an off-screen portion of the framebuffer.