The maximum size of a playfield display is determined by the maximum number of lines and the maximum number of columns. Vertically, the restrictions are simple. No data can be displayed in the vertical blanking area. The following table shows the allowable vertical display area. NTSC PAL ---- --- Vertical Blank Start 0 0 Vertical Blank Stop $15 (21) $1D (29) NTSC NTSC PAL PAL Normal Interlaced Normal Interlaced ------ ---------- ------ ---------- Displayable lines of screen video 241 483 283 567 =525-(21*2) =625-(29*2) Table 3-13: Maximum Allowable Vertical Screen Video Horizontally, the situation is similar. Strictly speaking, the hardware sets a rightmost limit to DDFSTOP of ($D8) and a leftmost limit to DDFSTRT of ($18). This gives a maximum of 25 words fetched in low resolution mode. In high resolution mode the maximum here is 49 words, because the rightmost limit remains ($D8) and only one word is fetched at this limit. However, horizontal blanking actually limits the displayable video to 368 low resolution pixels (23 words). These numbers are the same both for NTSC and for PAL. In addition, it should be noted that using a data-fetch start earlier than ($38) will disable some sprites . Table 3-14: Maximum Allowable Horizontal Screen Video Lores Hires ----- ----- DDFSTRT (standard) $0038 $003C DDFSTOP (standard) $00D0 $00D4 DDFSTRT (hw limits) $0018 $0018 DDFSTOP (hw limits) $00D8 $00D8 max words fetched 25 49 max display pixels 368 (low res) The limits on the display window starting and stopping positions described in this section apply to the Amiga's original custom chip set. In the Enhanced Chip Set (ECS), the limits for playfield display windows have been changed. For more information on ECS and playfield display windows refer to Appendix C, Enhanced Chip Set.
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