Routines
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9398: 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 9398 LD A,$EF Read from the keyboard;
Port Number Bit
0 1 2 3 4
EF 0 9 8 7 6
939A OUT ($FD),A
939C IN A,($FE)
939E LD C,A Keep the keys in C.
939F AND %00001000 Keep only bit 3 (up).
93A1 LD E,A Keep up in E.
93A2 LD A,C Fetch the keys.
93A3 RRCA Rotate A right twice.
93A4 RRCA
93A5 AND %01000101 Keep only bits 0, 2 and 6 (right, down and fire).
93A7 OR E Combine right, down and fire with up.
93A8 LD E,A Keep the keys so far in E.
93A9 RRCA Rotate A right twice, which moves fire to bit 4.
93AA RRCA
93AB AND %00010000 Keep only bit 4.
93AD OR E Combine fire with the keys so far.
93AE AND %00011111 Keep only bits 0-4.
93B0 LD E,A Keep the keys so far in E.
93B1 LD A,$F7 Read from the keyboard;
Port Number Bit
0 1 2 3 4
F7 1 2 3 4 5
93B3 OUT ($FD),A
93B5 IN A,($FE)
93B7 RRA Rotate A right three times, which moves left (5) to bit 1.
93B8 RRA
93B9 RRA
93BA AND %00000010 Keep only bit 1.
93BC OR E Combine left with the keys so far.
93BD RET Return.
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