PLA5 contains the flag_multiplexer, for testing the
TRUE condition.
it also generates the clock signals for loading the flags
during data calculation (step 3).
(Clock gating)
Note:
+ means, no transistor.
O means, transistor (BC847 or BC857) at this location.
S0-++++++++OO++++++
S1-++++++++OO++++++
!CMD13-O+O+O+O+++++++++
CMD13-+O+O+O+O++++++++
!CMD14-OO++OO++++++++++
CMD14-++OO++OO++++++++
!CMD15-OOOO++++++++++++
CMD15-++++OOOO++++++++
!N_FLAG-++O+++++++++++++
N_FLAG-+++O++++++++++++
!Z_FLAG-++++O+++++++++++
Z_FLAG-+++++O++++++++++
!C_FLAG-++++++O+++++++++
C_FLAG-+++++++O++++++++
INT-+O++++++++++++++
PHI21-++++++++OO++++++
CMD3-++++++++O+++++++
CMD6-++++++++OO++++++
||||||||||||||||
OOOOOOOO++++++++-TRUE
++++++++O+++++++-LC
+++++++++O++++++-LNZ
++++++++++++++++-
++++++++++++++++-
++++++++++++++++-
++++++++++++++++-
++++++++++++++++-
++++++++++++++++-
++++++++++++++++-
Flag_multiplexer for testing TRUE condition
(if TRUE=1, execute instruction. if TRUE=0, skip instruction.)
CMD15..13
000 TRUE = 1
001 TRUE = INT_FLAG
010 TRUE = !N_FLAG
011 TRUE = N_FLAG
100 TRUE = !Z_FLAG
101 TRUE = Z_FLAG
110 TRUE = !C_FLAG
111 TRUE = C_FLAG
Looks like some sort of 74151, doesn't it ?
Loading the flags:
LC = PHI21 & S1 & S0 & CMD6 &CMD3 //Load Flag C
LNZ = PHI21 & S1 & S0 & CMD6 //Load Flag N,Z
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(c) Dieter Mueller 2005