74LS00:                     5   ___  
          1   ___     P2  >----!   \   6
 P1 >---o----!   \   3      4  !    )O---+
        ! 2  !    )O-----------!___/     ! 9   ___
        +----!___/          12  ___      +----!   \   8
                      A15 >----!   \  11   10 !    )O----> V
                            13 !    )O--------!___/
                      A14 >----!___/

 MUX* >------------------o-----------------------------+
                         !                             !
    V >--------------o---)-------------------------+   !
                     !   !                         !   !
   P2 >----------+   !   !       RFRSH >-------+   !   !
                 !9  !10 !11                   !9  !10 !11
               +------------+                +------------+
            12 ! S2  S1  S0 !             12 ! S2  S1  S0 !
       A14 >---!D7          !      R7 >--o---!D7          !
            13 !            !            !13 !            !
       A15 >---!D6          !            o---!D6          !
            14 !  74LS157   !            !14 !   74LS151  !
       V >-o---!D5  "A"     !            o---!D5   "B"    !
           !15 !            ! 5          !15 !            ! 5
           o---!D4        Y !----> M8    +---!D4        Y !----> M7
           !1  !            !             1  !            !
           o---!D3          !      "1" >-o---!D3          !
           !2  !  8:1  MUX  !            !2  !  8:1  MUX  !
           +---!D2          !            +---!D2          !
            3  !            !             3  !            !
       P0 >----!D1          !       A14 >----!D1          !
            4  !            !             4  !            !
       P1 >----!D0          !       A15 >----!D0          !
               ! G* GND VCC !                ! G* GND VCC !
               +------------+                +------------+
                 !7  !8  !16                   !7  !8  !16
                 o---+   !                     o---+   !
                 !       !                     !       !
                ---      A                    ---      A
                 - GND   V +5 V                - GND   V +5 V

               +------------+          ---- +------------+
            1  !            ! 8        A    !  > '00  !  !------O +5 V
     RFRSH >---!CLK-1    Q2D!---> R7   !    !  1______+  !---< A15
            6  !            !          !    !  +------+  !---< A14
           +-<-!Q1D         !          !    !  >'151-A!  !------> M8
           !14 !   74LS393  !          !    !  1------+  !---< P2
           +->-!CLK-2       !         2.4"  !  +------+  !---< P1
            2  !       DUAL !          !    !  >'151-B!  !------> M7
           +---!CLR1  4 BIT !          !    !  1------+  !---< P0
           !12 !    COUNTER !          !    !  +------+  !---< MUX*
           o---!CLR2        !          !    !  > '393 !  !---< RFRSH
           !   ! GND    VCC !          V    !  1------+  !------V GND
           !   +------------+          ---- +------------+
           !     !7      !14
           +-----o       !                  !            !
                 !       !                  !>---1.2"--->!
                ---      A                   BOARD LAYOUT
                 - GND   V +5 V            (COMPONENT SIDE)

    Tie pin 1 of all (8) RAM chips together and pull up to +5 V through a
1/8th watt 1K ohm resistor on the bottom of the board.  Wire circuit as shown
on a small piece of perf board (1.2" x 2.4") with low profile sockets and 30 ga.
wirewrap wire.  Solder connections to mother board as follows:

        Signal:            Connection:
        (Name)           (IC no. - pin no.)

         +5 V                U15-16
         Gnd                 U15-8

         A15                 U15-2
         A14                 U15-3

         MUX*                U15-1

         RFRSH               U37-12

         P0                  U10-13
         P1                  U10-11
         P2                  U10-10

         M7                  U5-9
         M8                  U5-1

(Note the major role of U15, one of the dynamic RAM address multiplexers.)


    Cut the trace on the top of the board from U15, pin 4 to resistor pack
AR2, pin 4.  Insert chips in circuit.  Reassemble the computer, leaving off
the top cover.  Computer should still run normally.  Secure the circuit over
U14 and U15 with double sticky foam tape.  Replace the 64 K RAM chips with
256 K chips.  Computer should still run normally, but with BIOS control of
banks also functional.  Replace the top cover.

    Banks are controlled by U10, port C, bits 4, 6, and 7.  These bits
can be controlled by writing the properly masked values to port 057H
or by sending bit set/reset command values to port 054H. Bit 4 is the
least significant bit of the bank no. and bit 7 the most.  Banks 0-4 are
available in the lower 48K of RAM.  Bank 5 is a repeat of bank 4 and banks
6 and 7 are repeats of bank 0 (this is a feature, not a bug).