Detecting a system board's memory and i/o board configuration on a running E10k domain
(are just CPU's are active on those boards?) 

Use the board_id command from the SSP.  See the board_id man page.

It gives the assembly numbers for the system board and both mezzanines, if 
present.

You can also use the post2obp structure via redx. This will report the 
presence/absence of memory & I/O mezzanines as well as the sizes of the memory 
chunks configured for a given domain. Details are at 
http://hes.west.sun.com/dhpg/xftt/redx_how_do_i.html#p2o

You can also explore the shdimm command in redx for another vantage point. 

redx> repeat 4 {
? shdimm 0 $loopcnt
? }
redx> repeat 4 {
}? shdimm 0 $loopcnt
}? }
DIMMs 0.0[7:0] = 6F 6F 6F 6F 6F 6F 6F 6F
        Type 6F:
          0F   Size/Org[4:0]        Type[4:0]      128 MB dimm / 1 GB Bank
           3   Speed[1:0]           Type[6:5]      60 ns
           0   Reserved             Type[7]        
DIMMs 0.1[7:0] = 6F 6F 6F 6F 6F 6F 6F 6F
        Type 6F:
          0F   Size/Org[4:0]        Type[4:0]      128 MB dimm / 1 GB Bank
           3   Speed[1:0]           Type[6:5]      60 ns
           0   Reserved             Type[7]        
DIMMs 0.2[7:0] = FF FF FF FF FF FF FF FF
        Type FF: Empty Socket
DIMMs 0.3[7:0] = FF FF FF FF FF FF FF FF
        Type FF: Empty Socket

# ./prtdiag
System Configuration:  Sun Microsystems  sun4u SUNW,Ultra-Enterprise-10000
System clock frequency: 100 MHz
Memory size: 4608 Megabytes
            CPU Units: Frequency Cache-Size Version
            A: MHz  MB  Impl. Mask  B: MHz  MB  Impl. Mask
            C: MHz  MB  Impl. Mask  D: MHz  MB  Impl. Mask
            ----------  ----- ----  ----------  ----- ----
Board 3:       400 4.0   11    9.0     400 4.0   11    9.0  
               400 4.0   11    9.0     400 4.0   11    9.0  
Board 8:       400 4.0   11    9.0     400 4.0   11    9.0  
               400 4.0   11    9.0     400 4.0   11    9.0  
Board12:       400 4.0   11    9.0                          
                                                            
           Memory Units: Size 
           0: MB   1: MB   2: MB   3: MB
           -----   -----   -----   ----- 
Board 3        0       0    1024    1024 
Board 8     1024    1024       0       0 
Board12        0       0     256     256 

======================IO Cards=========================================
Board 3, SBus0:
               SBus clock frequency: 25 MHz
               0: QLGC,isp/sd(block)    'QLGC,ISP1000'
               1: SUNW,socal/sf(scsi-3) '501-3060'
Board 3, SBus1:
               SBus clock frequency: 25 MHz
               0: SUNW,qfe              'SUNW,sbus-qfe'
                  SUNW,qfe              'SUNW,sbus-qfe'
                  SUNW,qfe              'SUNW,sbus-qfe'
                  SUNW,qfe              'SUNW,sbus-qfe'
               1: SUNW,socal/sf(scsi-3) '501-3060'
Board 8, SBus0:
               SBus clock frequency: 25 MHz
               0: SUNW,hme              'SUNW,501-2739'
                  SUNW,fas/sd(block)  
               1: fcaw                  'FCW'
Board 8, SBus1:
               SBus clock frequency: 25 MHz
               0: SUNW,socal/sf(scsi-3) '501-3060'
               1: fcaw                  'FCW'
Board12, SBus0:
               SBus clock frequency: 25 MHz
               0: SUNW,hme              'SUNW,501-2739'
                  SUNW,fas/sd(block)  
Board12, SBus1:
               SBus clock frequency: 25 MHz
               0: SUNW,socal/sf(scsi-3) '501-3060'

For diagnostic information, see /var/opt/SUNWssp/adm/chef/messages on the SSP.

Note: shdimm can crash a running domain. 
Use with extreme caution!