Save routine
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80F5: Save Back to Skool to tape
saver This is the program used to save the fast code block for Back to Skool. See 'BASIC saver POKEs' for a discussion of the two POKEs that appear in line 33 of the program.
The fast code block consists of two sections of data. The first section contains the 16573 bytes of data for addresses 4000-80BC. The second section contains 65535 bytes of data starting at address 80CB, moving forward in steps of 23 bytes, and ending at 809D.
Note that the last three bytes in the first section (addresses 80BA-80BC) are actually loaded into addresses 8086, 809D and 80B4 by the load routine. Note also that the last byte in the second section, for address 809D, is not loaded by the load routine.
80F5 LD HL,$81B4 81B4 is where the game starts after loading
80F8 LD ($5D19),HL Store this address where it will be popped off the stack by the load routine
80FB EXX Prepare BC' for saving the second section of data
80FC LD BC,$0017
80FF EXX
8100 LD IX,$4000 4000 to 80BC will be saved first
8104 LD DE,$40BC
8107 LD A,$FF A flag byte of 0xFF (the first byte saved) indicates a data block
8109 LD HL,$0C98 This constant will give a leader tone of about 2 seconds
810C EX AF,AF'
810D INC DE Adjust the length and start address to allow for the flag byte
810E DEC IX
8110 DI Disable interrupts
8111 LD A,$02 MIC on, border red
8113 LD B,A
First create the 2-second leader tone.
8114 DJNZ $8114
8116 OUT ($FE),A
8118 XOR $0F
811A LD B,$A4
811C DEC L
811D JR NZ,$8114
811F DEC B
8120 DEC H
8121 JP P,$8114
Then create the sync pulse.
8124 LD B,$2F
8126 DJNZ $8126
8128 OUT ($FE),A
812A LD A,$0D
812C LD B,$37
812E DJNZ $812E
8130 OUT ($FE),A
It's time to save the first byte, which will be the flag byte (0xFF).
8132 LD BC,$1A0E B=0x1A (timing constant), C=0x0E (MIC off, border yellow)
8135 EX AF,AF' A=0xFF
8136 LD L,A L=0xFF
8137 JP $8143 Jump forward to save the flag byte
This is the main byte-saving loop.
813A LD A,D In the analagous ROM routine, the second instruction here is 'OR E', to check whether it's time to save the last byte (the parity byte)
813B LD A,D
813C JR Z,$814A This jump (to save the parity byte) is never made
813E LD L,(IX+$00) Fetch the next byte to be saved
8141 LD A,H A=current parity byte
8142 XOR L Update this for the next byte to be saved
8143 LD H,A H=new parity byte
8144 LD A,$01 A=0x01 (MIC on, border blue)
8146 SCF Set the carry flag (which will act as the marker bit)
8147 JP $8161 Jump forward to save the byte
This section of code, if it were used, would save the parity byte.
814A LD L,H Pick up the parity byte in L
814B JR $8141 Save it
This is the bit-saving loop for the first section of data (4000-80BC).
814D LD A,C A=0x0E (MIC off, border yellow) for the second pass
814E BIT 7,B Set the zero flag to indicate that this is the second pass through the loop
8150 DJNZ $8150
8152 JR NC,$8158 Jump if we are saving a '0'
8154 LD B,$20
8156 DJNZ $8156
8158 OUT ($FE),A
815A LD B,$1D Set the timing constant for the second pass
815C JR NZ,$814D Jump back for the second pass if we've just done the first
815E DEC B
815F XOR A Clear the carry flag
8160 INC A A=0x01 (MIC on, border blue)
8161 RL L Move the bit to be saved into the carry flag, and the marker bit leftwards
8163 JP NZ,$8150 Jump unless we've saved all 8 bits of the byte
A byte from the first section (4000-80BC) has just been saved. Are there any more left?
8166 DEC DE Decrease the length counter
8167 INC IX Move to the next byte to be saved
8169 LD B,$10 Set the timing constant for the first bit of the next byte
816B LD A,$7F Return if the BREAK key is being pressed
816D IN A,($FE)
816F RRA
8170 RET NC
8171 LD A,D Have we saved 4000 to 80BC yet?
8172 INC A
8173 JP NZ,$813A Jump back if not
Now a further 65535 bytes are saved: starting at 80CB, moving forward in steps of 23 bytes, and ending at 809D.
8176 LD A,D IX=80BD and DE=FFFF the first time we get here
8177 OR E
8178 JR Z,$8186 Jump if we have now saved 80CB onwards
817A LD L,(IX+$0E) IX+0x0E=80CB the first time we get here
817D LD A,H A=current parity byte
817E XOR L Update this for the next byte to be saved
817F LD H,A H=new parity byte
8180 LD A,$01 A=0x01 (MIC on, border blue)
8182 SCF Set the carry flag (which will act as the marker bit)
8183 JP $819D Jump forward to save the byte
This is where we come when all 82108 bytes have been saved.
8186 LD L,$00
8188 RET
This is the bit-saving loop for the second section of data.
8189 LD A,C A=0x0E (MIC off, border yellow) for the second pass
818A BIT 7,B Set the zero flag to indicate that this is the second pass through the loop
818C DJNZ $818C
818E JR NC,$8194 Jump if we are saving a '0'
8190 LD B,$20
8192 DJNZ $8192
8194 OUT ($FE),A
8196 LD B,$1D Set the timing constant for the second pass
8198 JR NZ,$8189 Jump back for the second pass if we've just done the first
819A DEC B
819B XOR A Clear the carry flag
819C INC A A=0x01 (MIC on, border blue)
819D RL L Move the bit to be saved into the carry flag, and the marker bit leftwards
819F JP NZ,$818C Jump unless we've saved all 8 bits of the byte
A byte from the second section has just been saved. Are there any more left?
81A2 DEC DE Decrease the length counter
81A3 EXX Move forward 23 bytes to the next byte to be saved
81A4 ADD IX,BC
81A6 EXX
81A7 LD B,$10 Set the timing constant for the first bit of the next byte
81A9 LD A,$7F These instructions check the BREAK key but do not act on the result
81AB IN A,($FE)
81AD RRA
81AE JP $8176 Save the next byte
The last 5 bytes of the first section saved make important changes to the load routine when the game is being loaded:
80B8 JR NZ,$807D This will replace the 'JR NZ,$808B' at 80B8, and kick off the loading of 65537 bytes from 8086 onwards (in steps of 23, all the way round to 8086 again)
80BA DEFB $20 This byte will replace the 0x31 at 8086, changing the instruction there from 'LD SP,$5D19' to 'JR NZ,$80A1'
80BB DEFB $DD This byte will be loaded into 809D (which already contains 0xDD)
80BC DEFB $AD This byte will be loaded into 80B4 (which already contains 0xAD)
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