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This set of routines can be readily divided into two parts:
  • i. The embedded colour item' handler.
  • ii. The 'colour system variable' handler.
i. Embedded colour items are handled by calling PRINT_A_1 as required.
A loop is entered to handle each item in turn. The entry point is at CO_TEMP_2.
CO_TEMP_1 21E1 RST $20 Consider the next character in the BASIC statement.
This entry point is used by the routine at CLASS_09.
CO_TEMP_2 21E2 CALL CO_TEMP_3 Jump forward to see if the present code represents an embedded 'temporary' colour item. Return carry set if not a colour item.
21E5 RET C
21E6 RST $18 Fetch the present character.
21E7 CP "," Jump back if it is either a ',' or a ';'; otherwise there has been an error.
21EB CP ";"
21EF JP REPORT_C Exit via 'report C'.
This entry point is used by the routine at PR_ITEM_1.
CO_TEMP_3 21F2 CP $D9 Return with the carry flag set if the code is not in the range +D9 to +DE (INK to OVER).
21F4 RET C
21F5 CP $DF
21F7 CCF
21F8 RET C
21F9 PUSH AF The colour item code is preserved whilst CH-ADD is advanced to address the parameter that follows it.
21FA RST $20
This entry point is used by the routine at CLASS_07.
The colour item code and the parameter are now 'printed' by calling PRINT_A_1 on two occasions.
CO_TEMP_4 21FC SUB $C9 The token range (+D9 to +DE) is reduced to the control character range (+10 to +15).
21FE PUSH AF The control character code is preserved whilst the parameter is moved to the calculator stack.
2202 POP AF
2203 AND A A return is made at this point if syntax is being checked.
2207 PUSH AF The control character code is preserved whilst the parameter is moved to the D register.
220B LD D,A
220D RST $10 The control character is sent out.
220E LD A,D Then the parameter is fetched and sent out before returning.
220F RST $10
2210 RET
This entry point is used by the routine at PO_TV_2.
ii. The colour system variables - ATTR-T, MASK-T and P-FLAG - are altered as required. On entry the control character code is in the A register and the parameter is in the D register.
Note that all changes are to the 'temporary' system variables.
CO_TEMP_5 2211 SUB $11 Reduce the range and jump forward with INK and PAPER.
2213 ADC A,$00
2215 JR Z,CO_TEMP_7
2217 SUB $02 Reduce the range once again and jump forward with FLASH and BRIGHT.
2219 ADC A,$00
The colour control code will now be +01 for INVERSE and +02 for OVER and the system variable P-FLAG is altered accordingly.
221D CP $01 Prepare to jump with OVER.
221F LD A,D Fetch the parameter.
2220 LD B,$01 Prepare the mask for OVER.
2222 JR NZ,CO_TEMP_6 Now jump.
2224 RLCA Bit 2 of the A register is to be reset for INVERSE 0 and set for INVERSE 1; the mask is to have bit 2 set.
2225 RLCA
2226 LD B,$04
CO_TEMP_6 2228 LD C,A Save the A register whilst the range is tested.
2229 LD A,D The correct range for INVERSE and OVER is only '0-1'.
222A CP $02
222E LD A,C Restore the A register.
222F LD HL,$5C91 It is P-FLAG that is to be changed.
2232 JR CO_CHANGE Exit via CO_CHANGE and alter P-FLAG using B as a mask, i.e. bit 0 for OVER and bit 2 for INVERSE.
PAPER and INK are dealt with by the following routine. On entry the carry flag is set for INK.
CO_TEMP_7 2234 LD A,D Fetch the parameter.
2235 LD B,$07 Prepare the mask for INK.
2237 JR C,CO_TEMP_8 Jump forward with INK.
2239 RLCA Multiply the parameter for PAPER by eight.
223C LD B,$38 Prepare the mask for PAPER.
CO_TEMP_8 223E LD C,A Save the parameter in the C register whilst the range of the parameter is tested.
223F LD A,D Fetch the original value.
2240 CP $0A Only allow PAPER/INK a range of '0' to '9'.
2242 JR C,CO_TEMP_9
This entry point is used by the routine at BORDER.
Report K - Invalid colour.
REPORT_K 2244 RST $08 Call the error handling routine.
2245 DEFB $13
Continue to handle PAPER and INK.
CO_TEMP_9 2246 LD HL,$5C8F Prepare to alter ATTR-T, MASK-T and P-FLAG.
2249 CP $08 Jump forward with PAPER/INK '0' to '7'.
224D LD A,(HL) Fetch the current value of ATTR-T and use it unchanged, by jumping forward, with PAPER/INK '8'.
2250 OR B But for PAPER/INK '9' the PAPER and INK colours have to be black and white.
2251 CPL
2252 AND $24
2254 JR Z,CO_TEMP_A Jump for black INK/PAPER, but continue for white INK/PAPER.
2256 LD A,B
CO_TEMP_A 2257 LD C,A Move the value to the C register.
The mask (B) and the value (C) are now used to change ATTR-T.
CO_TEMP_B 2258 LD A,C Move the value.
2259 CALL CO_CHANGE Now change ATTR-T as needed.
Next MASK-T is considered.
225C LD A,$07 The bits of MASK-T are set only when using PAPER/INK '8' or '9'.
225E CP D
225F SBC A,A
2260 CALL CO_CHANGE Now change MASK-T as needed.
Next P-FLAG is considered.
2263 RLCA The appropriate mask is built up in the B register in order to change bits 4 and 6 as necessary.
2264 RLCA
2265 AND $50
2267 LD B,A
2268 LD A,$08 The bits of P-FLAG are set only when using PAPER/INK '9'. Continue into CO_CHANGE to manipulate P-FLAG.
226A CP D
226B SBC A,A
The following subroutine is used to 'impress' upon a system variable the 'nature' of the bits in the A register. The B register holds a mask that shows which bits are to be 'copied over' from A to (HL).
CO_CHANGE 226C XOR (HL) The bits, specified by the mask in the B register, are changed in the value and the result goes to form the system variable.
226D AND B
226E XOR (HL)
226F LD (HL),A
2270 INC HL Move on to address the next system variable.
2271 LD A,B Return with the mask in the A register.
2272 RET
FLASH and BRIGHT are handled by the following routine.
CO_TEMP_C 2273 SBC A,A The zero flag will be set for BRIGHT.
2274 LD A,D The parameter is fetched and rotated.
2275 RRCA
2276 LD B,$80 Prepare the mask for FLASH.
2278 JR NZ,CO_TEMP_D Jump forward with FLASH.
227A RRCA Rotate an extra time and prepare the mask for BRIGHT.
227B LD B,$40
CO_TEMP_D 227D LD C,A Save the value in the C register.
227E LD A,D Fetch the parameter and test its range; only '0', '1' and '8' are allowable.
227F CP $08
2283 CP $02
The system variable ATTR-T can now be altered.
CO_TEMP_E 2287 LD A,C Fetch the value.
2288 LD HL,$5C8F This is ATTR-T.
228B CALL CO_CHANGE Now change the system variable.
The value in MASK-T is now considered.
228E LD A,C The value is fetched anew.
228F RRCA The set bit of FLASH/BRIGHT '8' (bit 3) is moved to bit 7 (for FLASH) or bit 6 (for BRIGHT).
2290 RRCA
2291 RRCA
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