annotate README @ 4:995d42b33974
serial output
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John Tsiombikas <nuclear@member.fsf.org> |
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Fri, 23 Feb 2018 13:29:37 +0200 |
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1 Amiga floppy boot test
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2 ----------------------
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3 Bootblock code in src/boot/boot.s uses the Amiga exec in ROM to load the
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4 flat binary image from from the first N sectors to address 0x10000, and jumps
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5 there. The size of the binary is patched immediately before the first
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6 instruction of the boot code by tools/mk_adf.py.
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7
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8 Linkscript puts code from the .startup segment (src/startup.s) at the load
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9 address, and it starts by setting the trap0 exception vector to point to the
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10 next instruction and executes a trap 0, to enter supervisor mode and take
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11 complete control of the system. Then it clears .bss, sets the stack and calls
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12 the C main function.
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13
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14 A few words about the graphics hack
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15 -----------------------------------
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16 The IFS fractal is calculated in 16.16 fixed point, using a mersenne twister
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17 random number generator (src/rng.c) for picking an IFS transformation to do each
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18 time. Two bitplanes are enabled both pointing to the same IFS pixel buffer, with
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19 a few scanlines offset and a different per-pixel scroll (data fetch delay) in
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20 BPLCON1. The palette is set up in a way to give priority to bitplane 0, i.e. use
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21 green for pixels with bit 0 set, and use a shadow color for pixels which have
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22 only bit 1 set. This gives the illusion of a shadow layer.
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23
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24 Serial debug output
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25 -------------------
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26 Serial comm needs RTS/CTS (hardware) flow control.
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27 stty 38400 crtscts cs8 -parenb -ixon -ixoff raw -iutf8 -F /dev/ttyUSB0
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