There are four addresses that access the beam position counter. Their usage is described in Table 7-5. Table 7-5: Contents of the Beam Position Counter VPOSR Read-only Read the high bit of the vertical position (V8) and the frame-type bit. Bit 15 LOF (Long-frame bit). Used to initialize interlaced displays. Bits 14-1 Unused Bit 0 High bit of the vertical position (V8). Allows PAL line counts (313) to appear in PAL versions of the Amiga. VHPOSR Read-only Read vertical and horizontal position of the counter that is producing the beam on the screen (also reads the light pen). Bits 15-8 Low bits of the vertical position, bits V7-V0 Bits 7-0 The horizontal position, bits H8-H1. Horizontal resolution is 1/160th of the screen width. VPOSW Write only Bits same as VPOSR above. VHPOSW Write only Bits same as VHPOSR above. Used for counter synchronization with chip test patterns. As usual, the address pairs VPOSR,VHPOSR and VPOSW,VHPOSW can be read from and written to as long words, with the most significant addresses being VPOSR and VPOSW.
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