Routines
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37784: Read Cursor Keys
Used by the routine at ReadControls.
Reads the cursor keys in the same way as the controls from ReadControls are returned. Bit 0 is right (8), bit 1 is left (5), bit 2 is down (6), bit 3 is up (7) and bit 4 is fire (0). A bit is reset when the key is pressed.
Output
A The state of the keys
ReadCursorKeys 37784 LD A,239 Read from the keyboard;
Port Number Bit
0 1 2 3 4
239 0 9 8 7 6
37786 OUT (253),A
37788 IN A,(254)
37790 LD C,A Keep the keys in C.
37791 AND %00001000 Keep only bit 3 (up).
37793 LD E,A Keep up in E.
37794 LD A,C Fetch the keys.
37795 RRCA Rotate A right twice.
37796 RRCA
37797 AND %01000101 Keep only bits 0, 2 and 6 (right, down and fire).
37799 OR E Combine right, down and fire with up.
37800 LD E,A Keep the keys so far in E.
37801 RRCA Rotate A right twice, which moves fire to bit 4.
37802 RRCA
37803 AND %00010000 Keep only bit 4.
37805 OR E Combine fire with the keys so far.
37806 AND %00011111 Keep only bits 0-4.
37808 LD E,A Keep the keys so far in E.
37809 LD A,247 Read from the keyboard;
Port Number Bit
0 1 2 3 4
247 1 2 3 4 5
37811 OUT (253),A
37813 IN A,(254)
37815 RRA Rotate A right three times, which moves left (5) to bit 1.
37816 RRA
37817 RRA
37818 AND %00000010 Keep only bit 1.
37820 OR E Combine left with the keys so far.
37821 RET Return.
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