No GetPC(), requires ESI=EIP
Assemble with Yasm

bits 32
b64decode:
    add esi, b64decode_end - _b64decode
    push    esi
    pop edi
b64_outer:
    push    04
    lodsd
    pop ecx
b64_inner:
    rol eax, 8
    cmp al, '0'
    jnb b64_testchar
    add al, (('/' << 2) + 1) & 0ffh
    shr al, 2 ;because '+' and '/' differ by only 1 bit
b64_testchar:
    add al, 4
    cmp al, 3fh
    jbe b64_store
    sub al, 45h
    cmp al, 19h
    jbe b64_store
    sub al, 6
b64_store:
    shrd    ebx, eax, 6
    loop    b64_inner
    xchg    ebx, eax
    bswap   eax
    stosd
    cmp byte [esi], '+'
    dec edi
    jnb b64_outer
b64decode_end:
    ;append your base64 data here
    ;terminate with printable character less than '+'