Pokémon Yellow credits routine

After the rating:

4F2E -> make the screen white

4F31 -> set C to 0x64
4F33 -> wait C frames (if called directly, C will be 0x00 and it waits 256 frames)

4F36 -> LCD stuff

4F39 -> set A to 0xA7
4F3B -> set x position of the window to A (if called directly, A will be 0x3B and everything will be off)

4F3D -> set A to 0x00
4F3E -> set "Background Horizontal Scrolling" to A
4F40 -> set "Background Vertical Scrolling" to A
4F42 -> set $FFAE to A
4F44 -> set $FFAF to A
4F46 -> set $FFB0 to A
4F48 -> set y position of the window to A

4F4A -> upload text graphics

4F4D -> set HL to 0xC3A0
4F50 -> set 0x4F bytes starting from HL to 0x7E (top black bar)
4F53 -> set HL to 0xC4B8
4F56 -> set 0x4F bytes starting from HL to 0x7E (bottom black bar)

4F59 -> set A to 0xC0
4F5B -> set the background palette to A

4F5D -> ??? (it has something to do with palettes and it works with values at around $DEE0 to $DFC0)

4F60 -> enable LCD operation (which was disabled at the end of $4F36)

4F63 -> sound stuff

4F66 -> set HL to 0x9C00
4F69 -> copy the screen in RAM into the window at (HL)
4F6C -> set HL to 0x9800
4F6F -> copy background (I guess?) in RAM into the background at (HL)

4F72 -> set C to 0x1F
4F74 -> set A to 0xC7
4F76 -> sound stuff (it also fills various parts at $C0xx with 0x00 and 0x01)

4F79 -> set C to 0x80
4F7B -> wait C frames

4F7E -> set A to 0x00
4F7F -> set $CD3D to A
4F82 -> set $CD3E to A
4F85 -> jump to $50B6

50B6 -> set DE to 0x5171
50B9 -> push DE

50BA -> pop DE
50BB -> set HL to 0xC421
50BE -> push HL
50BF -> make the screen white (copy 0x7F to $C3F0 - $C4B7)

50C2 -> pop HL

/ 50C3 -> set A to (DE)
| 50C4 -> DE += 1
| 50C5 -> push DE
|
| 50C6 - 50DC -> if A is 0xFF/0xFE/0xFD/0xFC/0xFB/0xFA then jump to $50EF/$50F6/$5100/$5107/$50E4/$510E
| 50DE -> get the next line to write (pointer is at (DE))
| 50E1 -> pop DE
\ 50E2 -> jump to $50C3

if 0xFB:
50E4:06 01     LD B,#01h
50E6:21 40 43  LD HL,#4340h
50E9:CD 84 3E  CALL #3E84h
50EC:D1        POP DE
50ED:18 D4     JR 50C3h

if 0xFF:
50EF:CD 88 4F  CALL #4F88h  <- 4F: LD C,A
50F2:0E 66     LD C,#66h
50F4:18 02     JR 50F8h

if 0xFE:
50F6:0E 7A     LD C,#7Ah
50F8:CD 2F 37  CALL #372Fh
50FB:CD A4 4F  CALL #4FA4h
50FE:18 BA     JR 50BAh

if 0xFD:
5100:CD 88 4F  CALL #4F88h
5103:0E 84     LD C,#84h
5105:18 02     JR 5109h

if 0xFC:
5107:0E 98     LD C,#98h
5109:CD 2F 37  CALL #372Fh
510C:18 AC     JR 50BAh

if 0xFA:
510E:CD 13 51  CALL #5113h
5111:D1        POP DE
5112:C9        RET

Our code directly jumps into a "special case" (normally tested at $50C6 - $50DC) at $50F1. This would explain the garbage screen at the start, because text is only updated at such a case.
It then goes into the normal loop where it reads the value at (DE). That value normally is set to 0x50B6, but we skipped that part.
Our DE is set to 0x0D5D just before the jump to $50F1 and it isn't changed after that (only +1 at $50C4 to get the next line of text).
So wrong values are loaded, but remember, the game waits for a "special case" to update text. The first value starting from $0D5D which gives a special case is 0xFA, which is the "THE END" case.
Now the code jumps to $510E and executes the normal code for the "THE END" screen.
After the return at $5112 the game eventually reaches $0D4C.
It accesses the bank pointer at $D35D and jumps to the word stored in $D36D.
In our case this is bank 0x3C and address $50F1.
Since just before the jump DE is set to 0x0D5D, it results in an infinite loop, which always shows "THE END".