The reverse of 7600: moves the lower window's rows down by one character row, working from the bottom. Used when a backspace goes back past the start of a line of typing (75AD).
763D
PUSH HL
Save the registers this uses
763E
PUSH DE
763F
PUSH BC
7640
PUSH AF
7641
LD HL,$50C0
HL = the third character row of the input window...
7644
LD DE,$50E0
...DE = the fourth, where it is moving to
7647
LD A,$05
Five pixel-line moves are needed to shift everything down one row
7649
PUSH HL
Save the row addresses
764A
PUSH DE
764B
LD B,$08
Eight pixel lines per character row
764D
PUSH HL
Save this pixel line's addresses
764E
PUSH DE
764F
PUSH BC
7650
LD BC,$0020
32 bytes per pixel line
7653
LDIR
Copy it down
7655
POP BC
Restore the counters
7656
POP DE
7657
POP HL
7658
INC H
Move down to the next pixel line
7659
INC D
765A
DJNZ $764D
One fewer pixel line to do
765C
POP DE
Restore the row addresses
765D
POP HL
765E
LD BC,$FFE0
Move both addresses BACK one character row instead (a negative step)