Load routines
Prev: 32768 Up: Map
32815: Fast load the main code block
Continues from 23831.
32815 LD SP,33020 Put the stack somewhere safe
32818 LD IX,16384 The fast code block starts loading at address 16384 (the start of the display file)
32822 LD DE,49152 Set DE (the byte counter) to a high enough value that it won't reach 0 before the first 16570 bytes (16384-32953) have been loaded
32825 LD A,255 We expect the first byte loaded (the flag byte) to be 255
32827 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
32828 INC D Reset the zero flag, indicating that we haven't loaded the first byte of the data block (the flag byte) yet
32829 EX AF,AF' Save these flags
32830 DEC D Restore the value of DE
32831 DI Disable interrupts
32832 LD A,15 BORDER 7
32834 OUT (254),A
32836 IN A,(254) Collect an initial EAR port reading into bit 6 of A
32838 RRA Move it to bit 5 of A
32839 AND 32 Clear the extraneous bits (0-4 and 6-7)
32841 OR 2 The border will turn red when the first edge is found
32843 LD C,A C will hold the border colour
32844 CP A Set the zero flag to avoid returning at the next instruction
32845 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
32846 CALL 32783 Listen for an edge
32849 JR NC,32845 Jump back to listen again if no edge was found within the time limit
An edge was found. Wait a bit and then listen again.
32851 LD HL,1045 Wait for about one second
32854 DJNZ 32854
32856 DEC HL
32857 LD A,H
32858 OR L
32859 JR NZ,32854
32861 CALL 32768 Are the edges still coming?
32864 JR NC,32845 Jump back if not
Check whether the signal is a leader tone.
32866 LD B,156 256 double edges arriving within a specific time limit constitute a valid leader tone
32868 CALL 32768
32871 JR NC,32845
32873 LD A,198
32875 CP B
32876 JR NC,32846
32878 INC H
32879 JR NZ,32866
This looks like a leader tone. Now listen for the first edge of the data block.
32881 LD B,201 Is the leader tone still there?
32883 CALL 32783
32886 JR NC,32845 Jump back if not
32888 LD A,B Have we found the first edge of the data block?
32889 CP 212
32891 JR NC,32881 Jump back if not
The first edge of the data block has been detected.
32893 CALL 32783 Look for the second edge of the data block
32896 RET NC Reset the Spectrum if it can't be found
Prepare to load the data block.
32897 LD A,C The border will alternate between blue and yellow for the data block
32898 XOR 3
32900 LD C,A
32901 LD H,0 Initialise the parity byte to 0
32903 LD B,225 Set the timing constant for the flag byte
32905 JR 32931 Jump forward to load the flag byte
This is the byte-loading loop. The first byte loaded is the flag byte.
32907 EX AF,AF' Restore the flags
32908 JR NZ,32915 Jump if the first byte (the flag byte) has just been collected
32910 LD (IX+0),L Load the byte read from tape into memory
32913 JR 32925
32915 RL C Save the carry flag in bit 0 of C temporarily
32917 XOR L L=first byte of the data block (the flag byte)
32918 RET NZ Reset the Spectrum if it wasn't 255
32919 LD A,C Restore the carry flag
32920 RRA
32921 LD C,A Restore C
32922 INC DE Compensate for the 'DEC DE' below
32923 JR 32927 Jump forward to start loading bytes into memory
32925 INC IX IX=next address to load the byte from tape into
32927 DEC DE Decrease the byte counter
32928 EX AF,AF' Save the flags (the zero flag is now set)
This inner loop loads the eight bits of a byte one-by-one from the tape into the L register.
32929 LD B,227 Set the timing constant
32931 LD L,1 Get ready to load eight bits from the tape
32933 CALL 32768 Load one bit from the tape
32936 RET NC Reset the Spectrum if there was a loading error
32937 LD A,237 Set the carry flag if a '1' was read from the tape, or reset it if a '0' was read
32939 CP B
32940 RL L Move the bit into the L register
32942 LD B,225 Set the timing constant for the next bit
32944 JP NC,32933 Jump unless eight bits have been loaded
A full byte has just been read from the tape.
32947 LD A,H Update the (ultimately unused) parity byte against the byte just read from the tape
32948 XOR L
32949 LD H,A
32950 LD A,D Set the zero flag if the the byte counter has reached 0 (which never happens)
32951 OR E
32952 JR NZ,32907 Jump back to load another byte from the tape
32954 RET (We never get here)
When the computer has loaded up to 32953, the instruction at 32952 is changed thus:
32952 JR NZ,32893
At this stage, 32893 reads as follows:
32893 LD (IX-52),L Effectively LD (32902),32
32896 EXX
32897 ADD IX,BC Add 23 to IX
32899 EXX
32900 SET 7,L
The instruction at 32893 above changes the instruction at 32902 from 'LD SP,23833' to:
32902 JR NZ,32929 Jump forward to load the next byte from the tape
Now 65536 more bytes are loaded, the last of which is at 32902 (even though there is one more byte, 32925, left on the tape: see the save routine). Then 32902 reads as follows:
32902 LD SP,23833 Point the stack pointer at the game start address that was placed at 23833 by the routine at 32484
32905 LD D,H
32906 RET To (23833)=24288
Prev: 32768 Up: Map