HORIZONTAL SCROLLING
You can scroll playfields horizontally from left to right or right to left on the screen. You
control the speed of scrolling by specifying the amount of delay in pixels. Delay means
that an extra word of data is fetched but not immediately displayed. The extra word is
placed just to the left of the window's leftmost edge and before normal data fetch. As the
display shifts to the right, the bits in this extra word appear on-screen at the left-hand
side of the window as bits on the right-hand side disappear off-screen. For each pixel of
delay, the on-screen data shifts one pixel to the right each display field. The greater the
delay, the greater the speed of scrolling. You can have up to 15 pixels of delay. In high-
resolution mode, scrolling is in increments of 2 pixels. Figure 3-24 shows how the delay
and extra data fetch combine to cause the scrolling effect.
To set up a playfield for horizontal scrolling, you need to;
o Define bit-planes wide enough to allow for the scrolling you need.
o Set the data-fetch registers to correctly place each horizontal line, including the extra
word, on the screen.
o Set the delay bits.
o Set the modulo so that the bit-plane pointers begin at the correct word for each line.
o Write Copper instructions to handle the changes during the vertical blanking interval.
SPECIFYING DATA FETCH IN HORIZONTAL SCROLLING
The normal data-fetch start for non-scrolled displays is ($38). If horizontal scrolling is
desired, then the data fetch must start one word sooner (DDFSTRT = $0030).
Incidentally, is will disable sprite 7. DDFSTOP remains unchanged. Remember that the
settings of the data-fetch registers affect both playfields.
SPECIFYING THE MODULO IN HORIZONTAL SCROLLING
As always, the modulo is two counts less than the difference between the address of the
next word you want to fetch and the address of the last word that was fetched. As an
example for horizontal scrolling, let us assume a 40-byte display in an 80-byte "big
picture." Because horizontal scrolling requires a data fetch of two extra bytes, the data for
each line will be 42 bytes long.
- Playfield hardware 77 -
Summary of Contents for Amiga A1000
Page 1: ...AMIGA HARDWARE REFERENCE MANUAL 1992 Commodore Business Machines Amiga 1200 PAL...
Page 20: ...Figure 1 1 Block Diagram for the Amiga Computer Family Introduction 11...
Page 21: ...12 Introduction...
Page 72: ...Figure 3 12 A dual Playfield display Playfield Hardware 63...
Page 87: ...Figure 3 24 Horizontal Scrolling 78 playfield hardware...
Page 101: ...92 Playfield Hardware...
Page 199: ...Figure 6 9 DMA time slot allocation 190 Blitter hardware...
Page 203: ...Figure 6 13 Blitter Block Diagram 194 Blitter Hardware...
Page 229: ...220 System Control Hardware...
Page 246: ...Figure 8 8 Chinon Timing diagram cont Interface Hardware 237...
Page 265: ...256 Interface Hardware...
Page 289: ...280 Appendix A...
Page 297: ...288 Appendix B...
Page 298: ...APPENDIX C CUSTOM CHIP PIN ALLOCATION LIST NOTE Means an active low signal Appendix C 289...
Page 302: ...APPENDIX D SYSTEM MEMORY MAP Appendix D 293...
Page 343: ...334 Appendix F...
Page 351: ...342 Appendix G...
Page 361: ...352 Appendix H...
Page 367: ...358 Appendix I...
