This variation of the Base64 encodeing sytem is slightly cheaper to code than the standard due to the rearranging of the values used.

6 bit input value	ASCII character range produced	gap
0...9	"0"..."9"	30h, 48d, '0'
10...35	"A"..."Z"	7, 'A'-':'
36...61	"a"..."z"	6, 'a'-'['
62	"+"	-80d, -50h, '+'-'{'
63	"?"	20d, 14h, '?'-','

Encode 3 bytes into 4 bytes consisting of only the characters 0...9, A...Z, a...z, and +

Entry:

• esi points to input buffer
• esi points to output buffer

lodsd
dec esi
mov edx,eax
mov ecx,4
.Repeat
	and al, 00111111y
	add al, ('0')
	.If al > '9'
		add al, ('A' - ':')
		.If al > 'Z'
			add al, ('a' - '[')
			.If al > 'z'
				.if al > 'z'+1
					mov al, '?'
				.else
					mov al, '+'
					.EndIf
				.EndIf
			.EndIf
		.EndIf

	stosb
	shr edx, 6
	mov al,dl
	.Untilcxz

Decode

	get value
		.if al < 'a'
		.if al < 'A'
			.if al < '0'
				.if al == '+'
					mov al, 62
				.else
					stc
					breakpoint
					.endif
			.elseif al > '9'
				.if al == '?'
					mov al, 63
				.else
					stc
					breakpoint
					.endif
			.else
				;0...9
				sub al, '0'
				.endif
		.elseif al > 'Z'
			stc
			breakpoint
		.else
			;A...Z
			sub al, 'A' - 10
			.endif
	.elseif al > 'z'
			stc
			breakpoint
	.else
		;a...z
		sub al, 'a' - 36
		.EndIf

Better?

	sub al, '0'
	.if Carry?
		.if al == '+' - '0'
			mov al, 62
		.else
			breakpoint ;invalid character in Mime input stream
			.endif
	.else
		.if al > 9
			sub al, 'A' - ':' ; if it was A, now its 10
			.if Carry?
				.if al == '?' - ('A' - '0')
					mov al, 63
				.else
					breakpoint ;invalid character in Mime input stream
					.endif
			.else
				.if al > 9 + 26
					sub al, 'a' - '[' ; if it was a, now its 36
					.if al > 9 + 26 + 26
						breakpoint ;invalid character in Mime input stream
						.endif
					.endif
				.endif
			.endif
		.endif