Routines
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0069: NMI_ISR: Non-Maskable Interrupt service routine
Used by the routines at NMI_VECTOR and NMI_JUMP.
NMI_ISR 0069 EXX Switch to shadow register set.
006A EX AF,AF'
006B PUSH HL Preserve HL on the stack (i.e. HL').
006C LD A,(ignore_packets_flag) If ignore_packets_flag is set jump to all_packets_read.
006F AND A
0070 JP NZ,all_packets_read
0073 XOR A Else clear field_header_flag.
0074 LD (field_header_flag),A
0077 LD HL,decoder_RAM Set HL to start of packet buffer in SRAM and store in packet_data_pointer
007A LD (packet_data_pointer),HL
007D LD A,(page_found_flag) If page_found_flag is set jump to row_search.
0080 AND A
0081 JP NZ,row_search
header_search 0084 LD A,$27 If byte at address in HL equals $27 (teletext framing code) jump forwards to packet_found.
0086 CP (HL)
0087 JR Z,packet_found
This entry point is used by the routine at store_packet_30.
not_header 0089 LD HL,(packet_data_pointer) Add 64 to address in packet_data_pointer.
008C LD DE,$0040
008F ADD HL,DE
0090 LD (packet_data_pointer),HL
0093 LD A,$24 Loop back to header_search until high byte of address equals $24 (loop over 1kB of decoder RAM).
0095 CP H
0096 JR NZ,header_search
0098 JP all_packets_read Jump to all_packets_read.
packet_found 009B LD (HL),$00 Overwrite framing code with 0 and increment HL (point to first byte of packet address).
009D INC HL
009E CALL decode_packet_number Get packet number (HL now points to second byte of packet address).
00A1 LD A,$1E If packet number equals 30, jump to store_packet_30.
00A3 CP B
00A4 JP Z,store_packet_30
00A7 XOR A Else if packet number is not 0, jump back to not_header.
00A8 CP B
00A9 JR NZ,not_header
00AB DEC HL Else decrement HL (point at first byte of packet address).
00AC CALL decode_magazine_number Get magazine number.
00AF LD A,(magazine_number) If magazine number is not magazine_number, jump to wrong_magazine.
00B2 CP B
00B3 JP NZ,wrong_magazine
00B6 INC HL Point HL at page tens.
00B7 INC HL
00B8 INC HL
00B9 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
00BC LD A,(page_tens) If decoded nibble is not equal to page_tens, jump to wrong_page.
00BF CP B
00C0 JP NZ,wrong_page
00C3 DEC HL Else decrement HL (point to page units).
00C4 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
00C7 LD A,(page_units) If decoded nibble is not equal to page_units, jump to wrong_page.
00CA CP B
00CB JP NZ,wrong_page
00CE LD A,(subpage_search_flag) If subpage_search_flag is clear jump to ignore_subcode.
00D1 AND A
00D2 JP Z,ignore_subcode
00D5 INC HL Point HL at Subcode S1.
00D6 INC HL
00D7 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
00DA LD A,(subcode_S1) If decoded nibble is not equal to subcode_S1, jump to wrong_page.
00DD CP B
00DE JP NZ,wrong_page
00E1 INC HL Point HL at Subcode S2.
00E2 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
00E5 LD A,B If lower three bits of decoded nibble is not equal to subcode_S2, jump to wrong_page.
00E6 AND $07
00E8 LD B,A
00E9 LD A,(subcode_S2)
00EC CP B
00ED JP NZ,wrong_page
00F0 INC HL Point HL at Subcode S3.
00F1 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
00F4 LD A,(subcode_S3) If decoded nibble is not equal to subcode_S3, jump to wrong_page.
00F7 CP B
00F8 JP NZ,wrong_page
00FB INC HL Point HL at Subcode S4
00FC CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL
00FF LD A,B If lower two bits of decoded nibble is not equal to subcode_S4, jump to wrong_page.
0100 AND $03
0102 LD B,A
0103 LD A,(subcode_S4)
0106 CP B
0107 JP NZ,wrong_page
This entry point is used by the routine at ignore_subcode.
page_found 010A INC HL Point HL at Control Bits C7-C10.
010B CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
010E LD A,B If byte is corrupt, jump to copy_header_text.
010F INC B
0110 JR Z,copy_header_text
0112 BIT 3,A Else if C10 (inhibit display) is clear, jump to inhibit_display_clear.
0114 JP Z,inhibit_display_clear
0117 LD (inhibit_display),A Else store value in inhibit_display.
011A LD HL,exit_pointer_RET Set exit_pointer to exit_pointer_RET (address of RET instruction at end of hamming_decode_hl). This prevents exiting to basic when a page with C10 set is loaded.
011D LD (exit_pointer),HL
This entry point is used by the routine at inhibit_display_clear.
copy_header_text 0120 LD HL,(packet_data_pointer) Copy 32 bytes from packet_data_pointer + 11 to page_receive_buffer_pointer + 8 (the header text).
0123 LD DE,$000B
0126 ADD HL,DE
0127 EX DE,HL
0128 LD HL,(page_receive_buffer_pointer)
012B LD BC,$0008
012E ADD HL,BC
012F EX DE,HL
0130 LD BC,$0020
0133 LDIR
0135 LD A,(field_header_flag) If field_header_flag is set, a header for this page has already been recieved during this field. Jump to no_erase_page.
0138 AND A
0139 JR NZ,no_erase_page
013B LD A,$01 Else set field_header_flag.
013D LD (field_header_flag),A
0140 LD HL,(packet_data_pointer) Eet HL to packet_data_pointer + 6 (Subcode S2 and C4).
0143 LD BC,$0006
0146 ADD HL,BC
0147 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL
014A LD A,B If byte is corrupt, or C4 (erase page) is clear, jump to no_erase_page.
014B CP $FF
014D JR Z,no_erase_page
014F BIT 3,A
0151 JR Z,no_erase_page
erase_page 0153 LD HL,(page_receive_buffer_pointer) Fill 1080 bytes with $20 (space character) at page_receive_buffer_pointer + 40. (clear rows 1-27).
0156 LD BC,$0028
0159 ADD HL,BC
015A LD D,H
015B LD E,L
015C INC DE
015D LD BC,$0437
0160 LD (HL),$20
0162 LDIR
no_erase_page 0164 LD A,$01 Set page_found_flag and jump to next_packet.
0166 LD (page_found_flag),A
0169 JP next_packet
wrong_magazine 016C LD A,B If value in B (first nibble of packet address) is corrupt, jump to not_header.
016D CP $FF
016F JP Z,not_header
0172 LD HL,(page_display_buffer_pointer) Else if byte at address in page_display_buffer_pointer equals $43 ('C' character), jump to copy_rolling_header to aid tuning.
0175 LD A,$43
0177 CP (HL)
0178 JP Z,copy_rolling_header
017B LD HL,(packet_data_pointer) Set HL to packet_data_pointer + 10 (Control bits C11-C14).
017E LD BC,$000A
0181 ADD HL,BC
0182 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
0185 LD A,B If byte is corrupt, jump to not_header.
0186 CP $FF
0188 JP Z,not_header
018B BIT 0,A Else if C11 (magazine serial) is clear, jump to copy_rolling_clock.
018D JR Z,copy_rolling_clock
018F DEC HL Point HL at Control bits C7-C10.
0190 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
0193 LD A,B If byte is corrupt or C7 (suppress header) is clear, jump to copy_rolling_header.
0194 INC B
0195 JR Z,copy_rolling_header
0197 BIT 0,A
0199 JR Z,copy_rolling_header
019B JR copy_rolling_clock Else jump to copy_rolling_clock.
wrong_page 019D LD A,(disable_rolling_headers) If disable_rolling_headers is set, jump to copy_rolling_clock.
01A0 AND A
01A1 JR NZ,copy_rolling_clock
01A3 LD HL,(packet_data_pointer) Set HL to packet_data_pointer + 9 (Control bits C7-C10).
01A6 LD BC,$0009
01A9 ADD HL,BC
01AA CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
01AD LD A,B If byte is corrupt, or C7 (suppress header) is set, or C9 (interrupted sequence) is set, jump to not_header.
01AE CP $FF
01B0 JP Z,not_header
01B3 BIT 2,A
01B5 JP NZ,not_header
01B8 BIT 0,A
01BA JP NZ,not_header
copy_rolling_header 01BD LD HL,(packet_data_pointer) Copy 32 bytes from packet_data_pointer + 11 to page_receive_buffer_pointer + 8 (the header text).
01C0 LD DE,$000B
01C3 ADD HL,DE
01C4 EX DE,HL
01C5 LD HL,(page_receive_buffer_pointer)
01C8 LD BC,$0008
01CB ADD HL,BC
01CC EX DE,HL
01CD LD BC,$0020
01D0 LDIR
01D2 JP not_header Jump to not_header.
copy_rolling_clock 01D5 LD HL,(packet_data_pointer) Copy 8 bytes from packet_data_pointer + 35 to page_receive_buffer_pointer + 32 (the clock text).
01D8 LD DE,$0023
01DB ADD HL,DE
01DC EX DE,HL
01DD LD HL,(page_receive_buffer_pointer)
01E0 LD BC,$0020
01E3 ADD HL,BC
01E4 EX DE,HL
01E5 LD BC,$0008
01E8 LDIR
01EA JP not_header Jump to not_header.
row_search 01ED LD HL,(packet_data_pointer) If byte at address in packet_data_pointer is not $27, (teletext framing code) jump to next_packet.
01F0 LD A,$27
01F2 CP (HL)
01F3 JR NZ,next_packet
01F5 LD (HL),$00 Else overwrite framing code with 0 and increment HL (point to first byte of packet address).
01F7 INC HL
01F8 CALL decode_packet_number Get packet number (HL now points to second byte of packet address).
01FB XOR A If packet number equals zero, jump to header_found.
01FC CP B
01FD JR Z,header_found
01FF DEC HL Else decrement HL (point at first byte of packet address).
0200 CALL decode_magazine_number Get magazine number.
0203 LD A,(magazine_number) If magazine number is not equal to magazine_number, jump to next_packet.
0206 CP B
0207 JR NZ,next_packet
0209 PUSH HL Preserve HL.
020A CALL decode_packet_number Get packet number (HL now points to second byte .of packet address)
020D LD A,B If packet number is 0, or is greater than 30, jump to bad_row.
020E AND A
020F JR Z,bad_row
0211 CP $1F
0213 JR NC,bad_row
0215 ADD A,A Else multiply packet number by 40.
0216 ADD A,A
0217 ADD A,B
0218 LD H,$00
021A LD L,A
021B ADD HL,HL
021C ADD HL,HL
021D ADD HL,HL
021E LD DE,(page_receive_buffer_pointer) Add to address in page_receive_buffer_pointer and store result in DE.
0222 ADD HL,DE
0223 EX DE,HL
0224 POP HL Restore HL and move forward 2 (start of 40 byte packet data).
0225 INC HL
0226 INC HL
0227 LD BC,$0028 Copy 40 bytes to address in DE and jump to next_packet.
022A LDIR
022C JR next_packet
bad_row 022E POP HL Balance stack.
next_packet 022F LD HL,(packet_data_pointer) Add 64 to address in packet_data_pointer.
0232 LD DE,$0040
0235 ADD HL,DE
0236 LD (packet_data_pointer),HL
0239 LD A,$24 Loop back to row_search until high byte of address equals $24 (loop over 1kB of decoder RAM).
023B CP H
023C JR NZ,row_search
all_packets_read 023E CALL set_hardware_counter Set hardware line counter ready for next video field.
0241 POP HL Restore HL from stack (stored HL' register value).
0242 EXX Switch back to main register set and return from NMI service routine.
0243 EX AF,AF'
0244 RETN
header_found 0246 DEC HL Decrement HL (point at first byte of packet address).
0247 CALL decode_magazine_number Get magazine number.
024A LD A,(magazine_number) If magazine number is not equal to magazine_number, jump to next_packet.
024D CP B
024E JP NZ,next_packet
0251 INC HL Move HL forward by 2 (point to page units).
0252 INC HL
0253 CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
0256 LD A,(page_units) If page units is not equal to page_units, jump to next_header.
0259 CP B
025A JR NZ,next_header
025C INC HL Increment HL (point to page tens).
025D CALL hamming_decode_hl Hamming 8/4 decode byte at address in HL.
0260 LD A,(page_tens) If page units is not equal to page_tens, jump to next_header.
0263 CP B
0264 JR NZ,next_header
0266 LD HL,(packet_data_pointer) Else set HL to packet_data_pointer and jump to packet_found.
0269 JP packet_found
next_header 026C XOR A Clear page_found_flag.
026D LD (page_found_flag),A
0270 LD (reveal_flag),A Clear reveal_flag.
0273 INC A Set page_ready_flag.
0274 LD (page_ready_flag),A
0277 JP not_header Jump to not_header.
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