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F6EA: Main loop
The routine at F6D5 continues here.
F6EA LD HL,$7FE3 7FE3 holds the lesson clock
F6ED DEC (HL) Decrement the LSB
F6EE JR NZ,$F6F7 Jump if it's non-zero
F6F0 INC L Point HL at the MSB of the lesson clock
F6F1 DEC (HL) Decrement the MSB
F6F2 LD A,(HL) Copy the new MSB to A
F6F3 INC A Have we reached the end of the lesson?
F6F4 CALL Z,$F74D Ring the bell and start the next lesson if so
F6F7 CALL $F6B4 Move the characters, close any temporarily open doors, and make the nearest teacher give ERIC lines if he's up to something he shouldn't be
F6FA LD HL,$7FFB 7FFB holds ERIC's primary status flags
F6FD LD A,(HL) Pick these up in A
F6FE AND A Are any of the status flags set?
F6FF JR Z,$F706 Jump if not
F701 CALL $F7AD Deal with ERIC if any of the status flags at 7FFB are set
F704 JR $F733
No status flags at 7FFB are set, which means ERIC is standing, or midstride, or just about to finish a 'short action' (bending over, dropping a stinkbomb, firing the water pistol, or moving forward as if to kiss). Here we finish ERIC's stride if he's midstride, finish any action he's just about to finish, or check for keypresses.
F706 LD L,$F3 Decrement ERIC's main action timer at 7FF3
F708 DEC (HL)
F709 JR NZ,$F733 Jump unless it's 0
F70B DEC L Collect the mid-action timer from 7FF2 into A; it will be non-zero if ERIC is midstride or mid-action
F70C LD A,(HL)
F70D LD (HL),$00 Reset the mid-action timer to 0
F70F INC L Copy the previous contents of the mid-action timer into the main action timer at 7FF3
F710 LD (HL),A
F711 AND A Is ERIC midstride or mid-action?
F712 JR NZ,$F730 Jump if so
ERIC is neither midstride nor just about to finish a short action, so check the keyboard.
F714 CALL $F413 Check for keypresses
F717 JR Z,$F733 Jump if there haven't been any
F719 LD ($7FF1),A Store the offset of the last keypress in 7FF1
F71C LD H,$E5 HL will index the table of keypress handling routines at E550
F71E LD DE,$F733 Push the address F733 (see below) onto the stack so we return there after dealing with the keypress
F722 LD L,A Point HL at the appropriate entry
F723 LD C,(HL) Copy the address of the routine for dealing with the keypress into BC
F724 INC L
F725 LD B,(HL)
F726 PUSH BC Push this address onto the stack
F727 LD HL,$D200 Point HL at byte 0x00 of ERIC's buffer
F72A LD A,(HL) Collect ERIC's animatory state into A, and his coordinates into DE
F72C LD E,(HL)
F72E LD D,(HL)
F72F RET Make an indirect jump to the relevant keypress-handling routine, and then return to F733
ERIC is midstride, or just about to finish an action (such as firing the water pistol or catching a mouse). Make him finish the stride or action.
F730 CALL $6E00 Update ERIC's animatory state and location, update the SRB, and scroll the screen if necessary
Now that ERIC's movements have been dealt with, the main loop continues.
F733 CALL $62A0 Update the display
F736 LD HL,$7FF3 7FF3 holds ERIC's main action timer
F739 LD A,(HL) Pick this up in A
F73A AND A Did we check for keypresses on this pass?
F73B JR NZ,$F73F Jump if not
F73D LD (HL),$02 Otherwise reset ERIC's main action timer to 2
This next section of code ensures that we don't pass through the main loop more than once every 1/50th of a second.
F73F LD L,$D9 HL=7FD9 (which holds the LSB of the system variable FRAMES as it was when the last pass through the main loop was completed)
F741 LD A,($5C78) A=LSB of the system variable FRAMES, which is incremented every 1/50th of a second
F744 SUB (HL) Now A=0 if FRAMES hasn't been incremented since the last pass through the main loop
F745 CP $01 Was FRAMES incremented?
F747 JR C,$F741 Jump back if not to check again
F749 ADD A,(HL) Store the current value of the LSB of FRAMES at 7FD9
F74A LD (HL),A
F74B JR $F6EA Jump back to the start of the main loop
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