Search notes:

x$ksmmem

x$ksmmem exposes the value of every byte in the SGA. Therefore the number of bytes in x$ksmmem is equal to the size of the SGA as sound in x$ksmsgmem: 
with ksmmem as (
  select
     count(*) * 8                             size_b,
    (count(*) * 8 / 1024/1024)                size_mb,
     min(to_number(addr, 'xxxxxxxxxxxxxxxx')) min_addr,
     max(to_number(addr, 'xxxxxxxxxxxxxxxx')) max_addr,
     min(to_number(addr, 'xxxxxxxxxxxxxxxx')) + (count(*)-1) * 8 max_addr_calculated
  from
     x$ksmmem
)
select
   ksmsgmem.ksmsgmemval  "Fixed SGA Size",
   ksmmem.*
from
   ksmmem,
   x$ksmsgmem ksmsgmem
where
    ksmsgmem.ksmsgmemnam = 'Fixed SGA Size'
;

See also

x$ksmfsv provides the names of variables and the addresses of their values in the SGA.
fixed tables

Index