Function: Hex2Dec

Converts hexadecimal values to decimal. Supports numbers larger than 2⁶⁴ (0xFFFFFFFF'FFFFFFFF). This script utilizes pre-generated multiplication table. It does exactly the same what humans do to convert hexadecimal to decimal by hand. It grabs proper decimal values from the table (array) and saves it to a variable that I like to name add chain. As the assignment finishes, the script sums all the numbers from such an add chain.
Seven powers of 16 are summed internally, using the set /a command. Why not eight? Take a sheet of paper and find out (hint: cmd.exe supports 32-bit signed integers from the range of -2³¹ ... 2³¹-1). These 7 numbers are summed the way they become one number in the add chain. If a number is greater than 2⁶⁴, then the script calculates further multiples of 16, till "the end".
The latest stage of conversion is summing of values from the add chain by the LargeInt_add procedure.
I can not guarantee the following script is 100% correct, neither did I check what's the biggest number it could handle. The multiple table has been generated using... LargeInt_add.

@echo off
:h2d
echo.
set/p var=give hex value 
call:HexToDec var var0
echo.
echo 0x%var% --^> %var0%
goto h2d
:HexToDec
setlocal&set var=%% %1 %%&set var=%var: =%&call set var=%var%
set sum=0
set exp=-1
set base=16
set pow16_0_=0;0;0;0;0;0;0;0;0;0;0;0;0;0;0
set pow16_1_=16;256;4096;65536;1048576;16777216;268435456;4294967296;68719476736;1099511627776;17592186044416;281474976710656;4503599627370496;72057594037927936;1152921504606846976
set pow16_2_=32;512;8192;131072;2097152;33554432;536870912;8589934592;137438953472;2199023255552;35184372088832;562949953421312;9007199254740992;144115188075855872;2305843009213693952
set pow16_3_=48;768;12288;196608;3145728;50331648;805306368;12884901888;206158430208;3298534883328;52776558133248;844424930131968;13510798882111488;216172782113783808;3458764513820540928
set pow16_4_=64;1024;16384;262144;4194304;67108864;1073741824;17179869184;274877906944;4398046511104;70368744177664;1125899906842624;18014398509481984;288230376151711744;4611686018427387904
set pow16_5_=80;1280;20480;327680;5242880;83886080;1342177280;21474836480;343597383680;5497558138880;87960930222080;1407374883553280;22517998136852480;360287970189639680;5764607523034234880
set pow16_6_=96;1536;24576;393216;6291456;100663296;1610612736;25769803776;412316860416;6597069766656;105553116266496;1688849860263936;27021597764222976;432345564227567616;6917529027641081856
set pow16_7_=112;1792;28672;458752;7340032;117440512;1879048192;30064771072;481036337152;7696581394432;123145302310912;1970324836974592;31525197391593472;504403158265495552;8070450532247928832
set pow16_8_=128;2048;32768;524288;8388608;134217728;2147483648;34359738368;549755813888;8796093022208;140737488355328;2251799813685248;36028797018963968;576460752303423488;9223372036854775808
set pow16_9_=144;2304;36864;589824;9437184;150994944;2415919104;38654705664;618475290624;9895604649984;158329674399744;2533274790395904;40532396646334464;648518346341351424;10376293541461622784
set pow16_10_=160;2560;40960;655360;10485760;167772160;2684354560;42949672960;687194767360;10995116277760;175921860444160;2814749767106560;45035996273704960;720575940379279360;11529215046068469760
set pow16_11_=176;2816;45056;720896;11534336;184549376;2952790016;47244640256;755914244096;12094627905536;193514046488576;3096224743817216;49539595901075456;792633534417207296;12682136550675316736
set pow16_12_=192;3072;49152;786432;12582912;201326592;3221225472;51539607552;824633720832;13194139533312;211106232532992;3377699720527872;54043195528445952;864691128455135232;13835058055282163712
set pow16_13_=208;3328;53248;851968;13631488;218103808;3489660928;55834574848;893353197568;14293651161088;228698418577408;3659174697238528;58546795155816448;936748722493063168;14987979559889010688
set pow16_14_=224;3584;57344;917504;14680064;234881024;3758096384;60129542144;962072674304;15393162788864;246290604621824;3940649673949184;63050394783186944;1008806316530991104;16140901064495857664
set pow16_15_=240;3840;61440;983040;15728640;251658240;4026531840;64424509440;1030792151040;16492674416640;263882790666240;4222124650659840;67553994410557440;1080863910568919040;17293822569102704640
:loop_
if not defined _var (
 set _var=%var:~-1%
) else (
 set _var=%_var%;%var:~-1%
)
set var=%var:~0,-1%
if defined var (goto loop_)
for %%A in (%_var%) do (call:loop %%A)
set n_=0
for %%A in (%chain%) do (set/a n_+=1)
if %n_% EQU 0 (goto:hextodec_out)
if %n_% LEQ 7 (call:sum_lesser&for /f "tokens=2* delims=^=" %%G in ('set component_') do (set sum=%%G)) else (call:sum_bigger)
:hextodec_out
endlocal&set %2=%sum%&exit/b0
:sum_bigger
call:sum_lesser
set chain=%component_%;%chain%
:sum_bigger_
for /f "tokens=1,2* delims=;" %%A in ("%chain%") do (set component=%%A&if "%%C" =="" (set chain=%%B) else (set chain=%%B;%%C))
if not "%component:~0,1%"=="0" (call:LargeInt_add sum component sum)
if defined chain (goto sum_bigger_)
exit/b0
:sum_lesser
for /f "tokens=1,2,3,4,5,6,7 delims=;" %%A in ("%chain%") do (call:sum_smaller_components %%A %%B %%C %%D %%E %%F %%G)
exit/b0
:sum_smaller_components
set/a component_+=%1
for /f "tokens=2* delims=;" %%H in ("%chain%") do (set chain=%%H;%%I)
shift
if not "%1"=="" (goto sum_smaller_components)
exit/b0
:loop
set num_=%1
if /i "%num_%"=="A" (set num_=10)
if /i "%num_%"=="B" (set num_=11)
if /i "%num_%"=="C" (set num_=12)
if /i "%num_%"=="D" (set num_=13)
if /i "%num_%"=="E" (set num_=14)
if /i "%num_%"=="F" (set num_=15)
set/a exp+=1
if %exp% GTR 0 (
 if %exp% LSS 16 (
  for /f "tokens=2* delims=^=" %%A in ('set pow16_%num_%_') do (for /f "tokens=%exp% delims=;" %%C in ("%%A;%%B") do (set num=%%C))
 )
 if %exp% GTR 15 (
  call:further_multiplies
 )
) else (set num=%num_%)
if defined chain (
 set chain=%chain%;%num%
) else (set chain=%num%)
exit/b0

:further_multiplies
if not defined tokens (set tokens=15)
for /f "tokens=2* delims=^=" %%G in ('set pow16_1_') do (for /f "tokens=%tokens% delims=;" %%H in ("%%G") do set pow16=%%H)
call:LargeInt_mul pow16 base pow16
set pow16_1_=%pow16_1_%;%pow16%
set/a tokens+=1
if "%num_%"=="0" (set num=0) else (call:LargeInt_mul num_ pow16 num)
exit/b0


:handle_buff
if %buff% GTR 9 (set buff=%buff:~-1%&set carry=%buff:~0,1%) else (set carry=)
exit/b0
:LargeInt_mul
setlocal
set left=%% %1 %%
set left=%left: =%
call set left=%left%
set right=%% %2 %%
set right=%right: =%
call set right=%right%
if defined left (if defined right (goto lrgintmul))
set exitcode=1
goto lrgintmul_endproc
:lrgintmul
set upper_component=%left%
set lower_component=%right%
:lrgintmul_loop
set temp_component_l=
set temp_component_u=
set temp_component_l=%lower_component:~-1%
set temp_component_u=%upper_component:~-1%
if defined temp_component_l (
 if defined temp_component_u (
  set/a buff=%temp_component_l%*%temp_component_u%
  if defined carry (set/a buff+=%carry%)
  call:handle_buff
 )
)
if "%suffix_added%"=="false" (
 set subst=%buff%%subst%%suffix%
) else (
 set subst=%buff%%subst%
)
set suffix_added=true
set upper_component=%upper_component:~0,-1%
if not defined upper_component (
 if defined carry (
  set subst=%carry%%subst%&set carry=
 )
)
if not defined upper_component (
 if not defined lower_component (
  goto lrgintmul_opsucc
 )
 set add_chain=%add_chain%;%subst%
 set subst=
 set upper_component=%left%
 set lower_component=%lower_component:~0,-1%
 set suffix_added=false
 set suffix=%suffix%0
)
if defined lower_component (goto lrgintmul_loop)
set sum=0
for %%A in (%add_chain%) do (set component=%%A&call:LargeInt_add sum component sum)
:lrgintmul_opsucc
set exitcode=0
:lrgintmul_endproc
call:ThrowErrorLevel exitcode %exitcode%
endlocal&set %3=%sum%&exit/b%errorlevel%


:LargeInt_add
setlocal
set left=%% %1 %%
set left=%left: =%
call set left=%left%
set right=%% %2 %%
set right=%right: =%
call set right=%right%
if defined left (if defined right (goto lrgintadd))
set exitcode=1
goto lrgintadd_endproc
:lrgintadd
set buff=
set lastchar_l=
set lastchar_r=
if defined left (set lastchar_l=%left:~-1%)
if defined right (set lastchar_r=%right:~-1%)
if defined carry (set/a buff+=%carry%&set carry=)
if defined lastchar_l (set/a buff+=%lastchar_l%)
if defined lastchar_r (set/a buff+=%lastchar_r%)
if %buff% GTR 9 (
 set outvar=%buff:~-1%%outvar%
 set carry=%buff:~0,1%
) else (
 set outvar=%buff%%outvar%
)
if defined left (set left=%left:~0,-1%)
if defined right (set right=%right:~0,-1%)
if not defined left (if not defined right (set outvar=%carry%%outvar%&goto lrgintadd_opsucc))
goto lrgintadd
:lrgintadd_opsucc
set exitcode=0
:lrgintadd_endproc
call:ThrowErrorLevel exitcode %exitcode%
endlocal&set %3=%outvar%&exit/b%errorlevel%
:ThrowErrorLevel
if defined %1 set %1=
exit/b%2