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Routines |
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| 45999 | INC IX | Advance IX by one byte | ||||||||
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This entry point is used by the routines at 46284, 46295 and 46306.
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| 46001 | INC IX | Advance IX by one byte | ||||||||
| 46003 | LD A,(IX+0) | If first byte of graphic layout data entry is at least 236, i.e. IX points to an instruction, then jump to appropriate graphic layout routine | ||||||||
| 46006 | CP 236 | |||||||||
| 46008 | JP NC,46218 | |||||||||
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At this point, the graphic layout data pointed to by IX is graphical data, not an instruction. We only draw the graphics if they lie within the display area (i.e. have x- and y-coordinates less than 32 and 24 respectively, see trivia).
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| 46011 | LD A,H | If x-coordinate is greater than 31 or y-coordinate is greater than 23 then skip ahead to 46151 | ||||||||
| 46012 | CP 32 | |||||||||
| 46014 | JP NC,46151 | |||||||||
| 46017 | LD A,L | |||||||||
| 46018 | CP 24 | |||||||||
| 46020 | JP NC,46151 | |||||||||
| 46023 | PUSH HL | Store HL (H = x-coordinate, L = y-coordinate) | ||||||||
| 46024 | DEC B | Decrease value of do-not-recalculate-display-buffer-address flag | ||||||||
| 46025 | JP Z,46051 | If we don't need to recalculate current position in display buffer then skip ahead to 46051 | ||||||||
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If B was 1, then we need to load DE with the address of the current position in the primary display buffer.
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| 46028 | LD A,H | Load BC with three times x-coordinate and DE with double y-coordinate | ||||||||
| 46029 | ADD A,A | |||||||||
| 46030 | ADD A,H | |||||||||
| 46031 | LD C,A | |||||||||
| 46032 | LD A,L | |||||||||
| 46033 | ADD A,A | |||||||||
| 46034 | LD E,A | |||||||||
| 46035 | LD D,0 | |||||||||
| 46037 | LD B,D | |||||||||
| 46038 | LD HL,46170 | Load DE with multiple of 96 corresponding to y-coordinate | ||||||||
| 46041 | ADD HL,DE | |||||||||
| 46042 | LD E,(HL) | |||||||||
| 46043 | INC HL | |||||||||
| 46044 | LD D,(HL) | |||||||||
| 46045 | LD HL,0 | Load DE with address in primary display buffer corresponding to x- and y-coordinates | ||||||||
| 46048 | ADD HL,DE | |||||||||
| 46049 | ADD HL,BC | |||||||||
| 46050 | EX DE,HL | |||||||||
| 46051 | LD A,(DE) | Load A with graphic set index plus one from this entry in primary display buffer and if value is now zero (i.e. old value was 255 corresponding to a block outside dimensions of room) then skip ahead to 46150 | ||||||||
| 46052 | INC A | |||||||||
| 46053 | JP Z,46150 | |||||||||
| 46056 | LD A,(34268) | Load current graphic set index into this location in primary display buffer | ||||||||
| 46059 | LD (DE),A | |||||||||
| 46060 | INC DE | Advance to graphic index in current character block's entry in primary display buffer | ||||||||
| 46061 | LD A,(IX+0) | Load graphic index into current display buffer location | ||||||||
| 46064 | LD (DE),A | |||||||||
| 46065 | INC DE | Advance to next byte in current character block's display buffer entry (attribute) | ||||||||
| 46066 | LD A,(34270) | Load A with current attribute and if this is not zero then skip ahead to 46077 | ||||||||
| 46069 | OR A | |||||||||
| 46070 | JR NZ,46077 | |||||||||
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Current attribute is zero (i.e. not set) so graphic layout data contains attribute information
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| 46072 | INC IX | Advance IX to next byte in graphic layout data (attribute) and load into A | ||||||||
| 46074 | LD A,(IX+0) | |||||||||
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At this point, A holds an attribute value either from the graphic layout data, or the current attribute as stored at 34270. This attribute is to be applied to the current display buffer location.
IX (current position in graphic layout data) is pointing to either the graphic index (if the current attribute at 34270 is set) or the attribute data (if the current attribute is not set). In either case, IX+1 is the address of the cursor shift byte. Bit 7 of the cursor shift byte is the override-attribute flag.
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| 46077 | BIT 7,(IX+1) | If bit 7 (override-attribute flag) is set then skip ahead to 46111 | ||||||||
| 46081 | JP NZ,46111 | |||||||||
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Override-attribute flag reset
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| 46084 | LD L,A | If PAPER is not zero (black) then skip ahead to 46111 | ||||||||
| 46085 | AND 56 | |||||||||
| 46087 | LD A,L | |||||||||
| 46088 | JP NZ,46111 | |||||||||
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The instructions between 46091 and 46110 (inclusive) are executed only when the PAPER component of the attribute in A (and L) is black. If the preserve-INK flag is reset in the attribute value loaded (at 46098) from the display buffer, then the instruction at 46107 will leave the reset (i.e. black) PAPER bits in A. If the preserve-INK flag is set, then the instructions between 46104 and 46106 (inclusive) will shift the bits used for INK into the PAPER positions and this PAPER colour (originally INK colour) will be preserved when the instruction at 46107 is executed.
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| 46091 | AND 192 | Load B with preserve-INK and mirror flags | ||||||||
| 46093 | LD B,A | |||||||||
| 46094 | LD A,L | |||||||||
| 46095 | AND 63 | Reset preserve-INK and mirror flags | ||||||||
| 46097 | LD C,A | Load C with INK and PAPER bits | ||||||||
| 46098 | LD A,(DE) | Load value currently in display buffer into A and if preserve-INK flag is reset then skip ahead to 46107 | ||||||||
| 46099 | BIT 6,A | |||||||||
| 46101 | JP Z,46107 | |||||||||
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Preserve INK flag set in display buffer
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| 46104 | ADD A,A | Shift INK bits into PAPER bits | ||||||||
| 46105 | ADD A,A | |||||||||
| 46106 | ADD A,A | |||||||||
| 46107 | AND 56 | Reset all except PAPER bits | ||||||||
| 46109 | OR B | (Re)set preserve-INK and mirror flags as stored in B previously | ||||||||
| 46110 | OR C | Set INK bits as stored in C previously (PAPER was black) | ||||||||
| 46111 | LD (DE),A | Load attribute in A into display buffer | ||||||||
| 46112 | POP HL | Restore HL (H = x-coordinate, L = y-coordinate) | ||||||||
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Read cursor shift byte. The value n, encoded in bits 0-6 of this byte, control where the cursor to write to the primary display buffer is moved to after writing the current character block.
If n is less than 33 then we move the cursor down a character row, and left by (32-n) characters.
If n is 33 or greater then we move the cursor right by (n-32) characters.
Bit 7 is the override-attribute flag.
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| 46113 | LD A,(IX+1) | Load A with cursor shift byte and if this is 255 (end marker) then jump to 46418 (load HL with stored coordinates from 34281 and return from drawing) | ||||||||
| 46116 | CP 255 | |||||||||
| 46118 | JP Z,46418 | |||||||||
| 46121 | AND 127 | Reset bit 7 (override-attribute flag) | ||||||||
| 46123 | SUB 33 | Subtract 33, and if remaining value is 0 (i.e. original value was 33) then skip ahead to 46140, else if value was greater than 33 then skip ahead to 46132 | ||||||||
| 46125 | JP Z,46140 | |||||||||
| 46128 | JP NC,46132 | |||||||||
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Cursor shift byte was less than 33 (move the cursor down a character row, and left by (32-n) characters)
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| 46131 | INC L | Increase y-coordinate | ||||||||
| 46132 | INC A | Increase A (x-coordinate offset) and add to x-coordinate | ||||||||
| 46133 | ADD A,H | |||||||||
| 46134 | LD H,A | |||||||||
| 46135 | LD B,0 | Prepare to recalculate display buffer address as change in coordinates is non-trivial | ||||||||
| 46137 | JP 45999 | Advance graphic layout data pointer by two bytes and read / process next data | ||||||||
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Cursor shift byte was 33 (move the cursor right by one character)
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| 46140 | INC DE | Advance pointer to start of next entry in primary display buffer | ||||||||
| 46141 | INC H | Increase x-coordinate by one | ||||||||
| 46142 | JP Z,46135 | If x-coordinate is zero then jump back to 46135 (see trivia) | ||||||||
| 46145 | LD B,1 | Prepare to skip over recalculation of display buffer address as we moved right one character | ||||||||
| 46147 | JP 45999 | Advance graphic layout data pointer by two bytes and read / process next data | ||||||||
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First byte in primary display buffer entry for current position is 255 (i.e. outside dimensions of room)
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| 46150 | POP HL | Restore HL (H = x-coordinate to draw to, L = y-coordinate to draw to) | ||||||||
| 46151 | LD A,(34270) | Load A with current attribute and if not zero then skip ahead to 46160 | ||||||||
| 46154 | OR A | |||||||||
| 46155 | JP NZ,46160 | |||||||||
| 46158 | INC IX | Advance IX to next byte in graphic layout data (attribute) | ||||||||
| 46160 | LD A,(IX+1) | Load A with cursor shift byte, reset bit 7 (override-attribute flag), subtract 33 and jump back to 46128 to update current drawing position | ||||||||
| 46163 | AND 127 | |||||||||
| 46165 | SUB 33 | |||||||||
| 46167 | JP 46128 | |||||||||
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