| Chase H.Q. | Routines |
| Prev: ECF3 | Up: Map | Next: ED6D |
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Used by the routine at ED6D.
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O:D Key half-row number in bits 0..2, key in bits 3+ [or is it inverted?] or $FF if no keys pressed O:F Z clear if keys are pressed 47 = 00101111 = kkkkkrrr (k = key is 5, r = row is 7)
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| keyscan_all | ED4D | LD DE,$FF2F | D = flag/counter? (255 to start), E = initial key and row counters (47 to start) | |||||
| ED50 | LD BC,$FEFE | Set B to $FE (initial keyboard half-row selector) and C to $FE (keyboard port number) | ||||||
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Start loop.
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| ka_loop | ED53 | IN A,(C) | Read port $<BC> | |||||
| ED55 | CPL | Complement the value returned to change it from active-low to active-high | ||||||
| ED56 | AND $1F | Discard any non-key flags | ||||||
| ED58 | JR Z,keyscan_all_1 | Jump to next iteration if no keys were pressed | ||||||
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Keys were pressed.
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| ED5A | INC D | ?If D's 255 here we're okay. anything else causes an exit | ||||||
| ED5B | RET NZ | Return if D is non-zero | ||||||
| ED5C | LD H,A | Copy key flags to H | ||||||
| ED5D | LD A,E | Copy key and row counters to A | ||||||
| keyscan_all_0 | ED5E | SUB $08 | Decrement key counter bitfield in A | |||||
| ED60 | SRL H | Shift a key bit out of H into the carry flag | ||||||
| ED62 | JR NC,keyscan_all_0 | Loop until we hit a set bit (at least one must be set since the zero case is handled earlier) | ||||||
| ED64 | RET NZ | Return if additional bits are set | ||||||
| ED65 | LD D,A | Set return value in D | ||||||
| keyscan_all_1 | ED66 | DEC E | Decrement E | |||||
| ED67 | RLC B | Rotate the half-row selector ($FE -> $FD -> $FB -> .. -> $7F) | ||||||
| ED69 | JR C,ka_loop | ...loop until the zero bit shifts out (eight iterations) | ||||||
| ED6B | CP A | Set Z | ||||||
| ED6C | RET | |||||||
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