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Routines |
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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
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| 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... | |
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Now we have the base screen address; work out the exact address to return.
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| 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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