Routines
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60928: Read Cursor Keys
Used by the routine at ReadControls.
Reads the cursor keys and puts the state of the controls in B, to be completed by ReadControls_Bottom_Row (see ReadControls). The keys are 5 (left), 6 (down), 7 (up), 8 (right) and 0 (fire), and a bit is reset when its key is pressed.
Output
B The state of the controls
Read_Cursor_Keys 60928 LD A,239 Read from the keyboard;
Port Number Bit
0 1 2 3 4
239 0 9 8 7 6
60930 OUT (253),A
60932 IN A,(254)
60934 LD C,A Stash the state of the keys in C.
60935 AND %00001000 Keep only bit 3, which is the key 7 (up).
60937 LD E,A Store it in E.
60938 LD A,C Fetch the state of the keys.
60939 RRCA Rotate the keys right twice, which moves the key 0 into bit 6, the key 6 into bit 2 and the key 8 into bit 0.
60940 RRCA
60941 AND %01000101 Keep only those three bits.
60943 OR E Combine them with up.
60944 LD E,A Store the result in E.
60945 RRCA Rotate the result right twice, which moves fire from bit 6 into bit 4.
60946 RRCA
60947 AND %00010000 Keep only bit 4.
60949 OR E Combine it with the other directions.
60950 AND %00011111 Keep only the five low bits, which are right, down, up and fire (the bit for left is not set yet).
60952 LD E,A Store the result in E.
60953 LD A,247 Read from the keyboard;
Port Number Bit
0 1 2 3 4
247 1 2 3 4 5
60955 OUT (253),A
60957 IN A,(254)
60959 RRA Rotate the keys right three times, which moves the key 5 into bit 1.
60960 RRA
60961 RRA
60962 AND %00000010 Keep only that bit (left).
60964 OR E Combine it with the other controls.
60965 LD B,A Store the state of the controls in B.
60966 JP ReadControls_Bottom_Row Jump to ReadControls_Bottom_Row to read the bottom row of the keyboard.
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