Routines
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C203: Screen Calculation
Used by the routines at 9200, Yin_Yang_Copy, Print_String, BF13 and C101.
This routine works out which area of the screen we are in. Here are a few examples of usage; A=22 HL=4000 A=22 (restored from E) AND 07 == A=02 BC=0200 HL=4200 A=22 (restored from E) AND 38 == A=20 BC=20 BC=80 HL=4280
Screen_Calc C203 LD E,A Stash the accumulator in E for later.
C204 AND $C0 Mask off low bits d6-d7.
C206 JP Z,Screen_Top If the result is now zero, jump to Screen_Top.
C209 CP $40 If A is 40 then jump to Screen_Middle.
C20B JP Z,Screen_Middle
Screen_Bottom C20E LD HL,$5000 Sets HL to 5000 (the bottom of the screen buffer).
C211 JP Screen_Calc_Minor Jump to Screen_Calc_Minor.
Screen_Middle C214 LD HL,$4800 Sets HL to 4800 (the middle of the screen buffer).
C217 JP Screen_Calc_Minor Jump to Screen_Calc_Minor.
Screen_Top C21A LD HL,$4000 Sets HL to 4000 (the top of the screen buffer) and continue on...
Now we have the base screen address; work out the exact address to return.
Screen_Calc_Minor C21D LD A,E Restore the accumulator from E.
C21E AND $07 Mask bits d0-d2.
C220 LD B,A Push the accumulator into BC as the high-order byte (low is 00).
C221 LD C,$00
C223 ADD HL,BC Adds BC to HL.
C224 LD A,E Again, restore the accumulator from E.
C225 AND $38 Mask bits d3-d5.
C227 LD C,A Push the accumulator into BC as the low-order byte (high is 00).
C228 LD B,$00
C22A SLA C * 2 * 2.
C22C RL B
C22E SLA C
C230 RL B
C232 ADD HL,BC Adds BC to HL.
C233 RET Return.
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