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Load routines |
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Continues from 5D17.
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| 802F | LD SP,$80FC | Put the stack somewhere safe | ||
| 8032 | LD IX,$4000 | The fast code block starts loading at address 4000 (the start of the display file) | ||
| 8036 | LD DE,$C000 | Set DE (the byte counter) to a high enough value that it won't reach 0 before the first 16570 bytes (4000-80B9) have been loaded | ||
| 8039 | LD A,$FF | We expect the first byte loaded (the flag byte) to be 0xFF | ||
| 803B | SCF | In the analogous ROM routine, setting the carry flag would indicate that we want to LOAD rather than VERIFY; here, this flag is not used | ||
| 803C | INC D | Reset the zero flag, indicating that we haven't loaded the first byte of the data block (the flag byte) yet | ||
| 803D | EX AF,AF' | Save these flags | ||
| 803E | DEC D | Restore the value of DE | ||
| 803F | DI | Disable interrupts | ||
| 8040 | LD A,$0F | BORDER 7 | ||
| 8042 | OUT ($FE),A | |||
| 8044 | IN A,($FE) | Collect an initial EAR port reading into bit 6 of A | ||
| 8046 | RRA | Move it to bit 5 of A | ||
| 8047 | AND $20 | Clear the extraneous bits (0-4 and 6-7) | ||
| 8049 | OR $02 | The border will turn red when the first edge is found | ||
| 804B | LD C,A | C will hold the border colour | ||
| 804C | CP A | Set the zero flag to avoid returning at the next instruction | ||
| 804D | RET NZ | In the analogous ROM routine, this instruction would return if the BREAK key is being pressed; here, the zero flag is always set | ||
| 804E | CALL $800F | Listen for an edge | ||
| 8051 | JR NC,$804D | Jump back to listen again if no edge was found within the time limit | ||
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An edge was found. Wait a bit and then listen again.
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| 8053 | LD HL,$0415 | Wait for about one second | ||
| 8056 | DJNZ $8056 | |||
| 8058 | DEC HL | |||
| 8059 | LD A,H | |||
| 805A | OR L | |||
| 805B | JR NZ,$8056 | |||
| 805D | CALL $8000 | Are the edges still coming? | ||
| 8060 | JR NC,$804D | Jump back if not | ||
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Check whether the signal is a leader tone.
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| 8062 | LD B,$9C | 256 double edges arriving within a specific time limit constitute a valid leader tone | ||
| 8064 | CALL $8000 | |||
| 8067 | JR NC,$804D | |||
| 8069 | LD A,$C6 | |||
| 806B | CP B | |||
| 806C | JR NC,$804E | |||
| 806E | INC H | |||
| 806F | JR NZ,$8062 | |||
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This looks like a leader tone. Now listen for the first edge of the data block.
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| 8071 | LD B,$C9 | Is the leader tone still there? | ||
| 8073 | CALL $800F | |||
| 8076 | JR NC,$804D | Jump back if not | ||
| 8078 | LD A,B | Have we found the first edge of the data block? | ||
| 8079 | CP $D4 | |||
| 807B | JR NC,$8071 | Jump back if not | ||
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The first edge of the data block has been detected.
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| 807D | CALL $800F | Look for the second edge of the data block | ||
| 8080 | RET NC | Reset the Spectrum if it can't be found | ||
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Prepare to load the data block.
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| 8081 | LD A,C | The border will alternate between blue and yellow for the data block | ||
| 8082 | XOR $03 | |||
| 8084 | LD C,A | |||
| 8085 | LD H,$00 | Initialise the parity byte to 0 | ||
| 8087 | LD B,$E1 | Set the timing constant for the flag byte | ||
| 8089 | JR $80A3 | Jump forward to load the flag byte | ||
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This is the byte-loading loop. The first byte loaded is the flag byte.
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| 808B | EX AF,AF' | Restore the flags | ||
| 808C | JR NZ,$8093 | Jump if the first byte (the flag byte) has just been collected | ||
| 808E | LD (IX+$00),L | Load the byte read from tape into memory | ||
| 8091 | JR $809D | |||
| 8093 | RL C | Save the carry flag in bit 0 of C temporarily | ||
| 8095 | XOR L | L=first byte of the data block (the flag byte) | ||
| 8096 | RET NZ | Reset the Spectrum if it wasn't 0xFF | ||
| 8097 | LD A,C | Restore the carry flag | ||
| 8098 | RRA | |||
| 8099 | LD C,A | Restore C | ||
| 809A | INC DE | Compensate for the 'DEC DE' below | ||
| 809B | JR $809F | Jump forward to start loading bytes into memory | ||
| 809D | INC IX | IX=next address to load the byte from tape into | ||
| 809F | DEC DE | Decrease the byte counter | ||
| 80A0 | EX AF,AF' | Save the flags (the zero flag is now set) | ||
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This inner loop loads the eight bits of a byte one-by-one from the tape into the L register.
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| 80A1 | LD B,$E3 | Set the timing constant | ||
| 80A3 | LD L,$01 | Get ready to load eight bits from the tape | ||
| 80A5 | CALL $8000 | Load one bit from the tape | ||
| 80A8 | RET NC | Reset the Spectrum if there was a loading error | ||
| 80A9 | LD A,$ED | Set the carry flag if a '1' was read from the tape, or reset it if a '0' was read | ||
| 80AB | CP B | |||
| 80AC | RL L | Move the bit into the L register | ||
| 80AE | LD B,$E1 | Set the timing constant for the next bit | ||
| 80B0 | JP NC,$80A5 | Jump unless eight bits have been loaded | ||
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A full byte has just been read from the tape.
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| 80B3 | LD A,H | Update the (ultimately unused) parity byte against the byte just read from the tape | ||
| 80B4 | XOR L | |||
| 80B5 | LD H,A | |||
| 80B6 | LD A,D | Set the zero flag if the the byte counter has reached 0 (which never happens) | ||
| 80B7 | OR E | |||
| 80B8 | JR NZ,$808B | Jump back to load another byte from the tape | ||
| 80BA | RET | (We never get here) | ||
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When the computer has loaded up to 80B9, the instruction at 80B8 is changed thus:
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| 80B8 | JR NZ,$807D | |||
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At this stage, 807D reads as follows:
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| 807D | LD (IX-$34),L | Effectively LD ($8086),$20 | ||
| 8080 | EXX | |||
| 8081 | ADD IX,BC | Add 23 to IX | ||
| 8083 | EXX | |||
| 8084 | SET 7,L | |||
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The instruction at 807D above changes the instruction at 8086 from 'LD SP,$5D19' to:
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| 8086 | JR NZ,$80A1 | Jump forward to load the next byte from the tape | ||
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Now 65536 more bytes are loaded, the last of which is at 8086 (even though there is one more byte, 809D, left on the tape: see the save routine). Then 8086 reads as follows:
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| 8086 | LD SP,$5D19 | Point the stack pointer at the game start address that was placed at 5D19 by the save routine | ||
| 8089 | LD D,H | |||
| 808A | RET | To ($5D19)=81B4 | ||
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