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Used by the routines at PO_SCR, CL_SET, CL_SC_ALL and CL_LINE.
For a given line number, in the B register, the appropriate display file address is formed in the HL register pair.
B Line number
HL Display file address
CL_ADDR 03739 LD A,24 The line number has to be reversed.
03741 SUB B
03742 LD D,A The result is saved in D.
03743 RRCA In effect '(A mod 8)*32'. In a 'third' of the display the low byte for the first line is 0, for the second line 32, etc.
03744 RRCA
03745 RRCA
03746 AND 224
03748 LD L,A The low byte goes into L.
03749 LD A,D The true line number is fetched.
03750 AND 24 In effect '64+8*INT (A/8)'. For the upper 'third' of the display the high byte is 64, for the middle 'third' 72, and for the lower 'third' 80.
03752 OR 64
03754 LD H,A The high byte goes to H.
03755 RET Finished.
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