GRAND THEFT AUTO 1 - HOW TO *ACTUALLY* FIX THE PLAYER_A.DAT BUG IN THE ROCKSTAR CLASSICS / STEAM EDITION
Analysis and Fix by Daniel Marschall, 08 Feb 2019
Current release: https://misc.daniel-marschall.de/spiele/gta1/player_a_bugfix.txt

Problem:
	GTA1 Rockstar Classics or Steam Edition crashes after the first run.
	The reason is the resolution setting which is saved into player_a.dat ("fifth byte")
	The setting is actually correctly loaded, but the game
	will crash during the loading of the setting.
	
Existing Workarounds:
	Currently, the workaround is mainly to change the fifth byte to the value "0x06" (invalid)
	before running GTA. The tool GTAFixer.exe can do that for you.

	Unfortunately, with to this workaround, the resolution won't be kept when
	the game is launched again.
	
Actual problem:
	In function sub_414C70, following happens:
	The resolution is saved in "byte_00503224". The values are:
		00 = 640x480x16
		01 = 800x600x16
		02 = 1024x768x16
		03 = 640x480x32
		04 = 800x600x32
		05 = 1024x768x32
	It turns out that the function "sub_414C70" sets the variable like this:
		function sub_414C70(int desired_resolution) {
			...
			int x;
			if (... >= desired_resolution) {
				x = 1;
			} else {
				x = 0;
			}
			x--; // x is now either $00000000 or $FFFFFFFF
			byte_00503224 = x & 6; // byte_00503224 is now either 0 or 6
			                       // This is wrong! Resolution "6" does not exist! (Valid: 0..5)
			...
		}
	So, byte_00503224 will become either 0 or 6. If it becomes 6, the game will crash
	in function "sub_414CC0", because 6 is no valid resolution. It is a typical off-by-one bug.
	Actually, I do not understand this function, because actually we want to set
	byte_00503224 equal to the argument "desired_resolution", so that the resolution from
	the previous game session gets recovered?!
	
	To fix the problem, we change the code like this:
		function sub_414C70(int desired_resolution) {
			...
			byte_00503224 = desired_resolution;
			...
		}
	
	In assembly:	
		sub_414C70 proc near
		arg_0= dword ptr  4   ; arg_0 = desired_resolution
		mov     eax, dword_503214
		mov     ecx, dword_503218
		lea     edx, [eax+ecx*2]
		mov     ecx, dword_503210
		mov     eax, dword_50320C
		add     edx, ecx
		mov     ecx, dword_503208
		add     edx, eax
		mov     eax, dword_503204
		add     edx, ecx
		mov     ecx, [esp+arg_0]
		add     edx, eax
		xor     eax, eax
		mov     al, [ecx]     ; apply following patch:
		cmp     eax, edx      ; mov edx, eax
		setnl   dl            ; nop
		dec     dl            ; nop
		and     edx, 6        ; nop
		mov     byte ptr byte_00503224, dl
		retn
		sub_414C70 endp
		
	In the HexEditor, you need to apply following patch:
	
		Search:   3B C2 0F 9D C2 FE CA 83 E2 06
		Replace:  89 C2 90 90 90 90 90 90 90 90

	Now you can finally play the game without GTAFixer.exe ,
	and your resolution will be kept for the next game session.