                
		PDP-1 COMPUTER
		MODIFICATION BULLETIN no. 2

          The  PDP-1  computer now  is  equipped with  a  high speed
magnetic drum storage.  The drum is  divided into 22 fields of  4096
words  each.  Words  are transferred between  the drum  and the core
memory under  automatic  control.   Because  the  drum  runs  at  30
revolutions  per second,  each word  on the  drum is  available once
every 33-1/3  milliseconds_, but  when a  drum operation  has  begun,
words  are transferred at a rate  of 8.16 microseconds each.  In one
operation, information can  be written  on the drum,  read from  the
drum, or both written and read simultaneously. 
          Each field has a five bit address and each word in a given
field has a twelve bit address.   By use of the instructions  below,
the  programmer specifies a drum field,  an initial word address, an
initial core memory address, the number of words to be  transferred,
and  whether the operation  is to write  on the drum,  read from the
drum, or swap the contents of  core memory with the contents of  the
drum. 
          The  following instructions have been  added to the PDP-1_.
          DIA (iot 60)  - [drum  initial address]  Causes the  C(IO)
bits  1-5 to  be sent  to the drum  write field  buffer.  These bits
specify which field of the drum  will be written on during the  next
                
                                                        DCC instruction_, or, if C(IO) bits 1-5=0, that no write operation is
to occur.  The C(IO) bits 6-17 are sent to the drum initial  address
register to specify the first drum address to be transferred. 
          DBA (iot 61) [drum break on address] Causes the C(IO) bits
6-17 to be  sent to  the drum  initial address  register.  When  the
current  drum address becomes  equal to the  contents of the initial
address register a sequence  break request is  initiated.  Bit 5  of
the  status word is set by the break, and is cleared by the next DCC
instruction. 
          DCC (iot 62)  [drum count and  commence] Causes the  C(IO)
bits  1-5 to  be sent  to the  drum read  field buffer.   These bits
specify which field will be read_, or, if C(IO) bits 1-5 = 0, that no
read  operation is to occur.  The  C(AC) bits 6-17 specify the first
core memory address of the data  to be transferred.  The C(IO)  bits
6-17  specify the number  of words to be  transferred.  If the C(IO)
bits 6-17 = 0, 4096 words  are transferred.  While the DCC  instruc-
tion is being executed, the computer stops and the drum system takes
full control of the core  memory.  Successive words are  transferred
from  sequential locations until  the operation is  complete.  If no
errors occurred during the drum operation, the instruction following
the  DCC  is skipped.   The  C(AC) and  C(IO)  are lost  during this
operation. 
          If both the read  field and the  write field are  non-zero
(both  read  and write  operations  are specified)  the  contents of
memory are written on the write field_, then the read field data  are
                
                                                        read into memory.  The read field must not equal the write field. 
          In  order to avoid passing a given drum address, and hence
losing 33 milliseconds, the DCC  instruction must be given at  least
250  microseconds  before  the  drum  address  reaches  the  initial
address. 
          DRA (iot 63) [drum read  address] Causes the current  drum
address to be read into IO bits 6-17.  The parity error flag is read
into IO bit 0_, and the timing error flag is read into IO bit 2.  Two
cycles elapse before this information is placed in the IO.  

ERROR CONDITIONS














                
                                                        
          The drum will be a major component of the forthcoming PDP-
1 Time  Sharing System  described in  memo PDP-6.   Because  several
fields  will  be  needed  for the  time  sharing  system,  users are
requested to use the lower numbered fields when possible.  
      2             
                                                                                         
