The ALU and Basic Arithmetic
Last Edit November 1, 1996; May 1, 1999 ; July 15, 2001
Some operations will be examined fro the two-RALU slices, the Am2901
ans the Am2903. [AMD also did an Am29203 that was released after
this book was published.]
It is desired to increment a register and at the same time output
its original value:
R'A <-- RA + 1; RA -->
Assume that the register is B15 and that it has been
previously loaded with a value. If it has not been previously loaded
with a value, the contents are unknown and undetermined.
The Am2901 microcontrols are derived by examining each subfield
table. The A and B addresses will both be F16. The only
way to output from a scratchpad is to select A with the output MUX.
This leaves the B,0 pair as the source control.; therefore I2I1I0
is set at 38. The ALU function is add or increment; therefore
I5I4I3 is set to 08,
and CN is set to 18.
The result of the ADD is to be passed around from the ALU output
F to the RAM shift register and then to the scratchpad memory, to
R15. This is the destination control F--> B, A --> Y; therefore
I8I7I6 is set to 288.
The complete microword for the Am2901 is:
The data flow is given in Figure 6-3.
Figure 6-3 Am2901: Increment a register and output its original
The same problem approach is used with the Am2903. The only way
to output directly from the scratchpad is via the DBout
path, which requires OE'n be held LOW. The ALU will add
A + 0; therefore I4I3I2I1
will be set to 616, Cin set to 1, and I0
set to 0.
The A port input MUX select E'A is set LOW. The A and
B addresses are set to F16, assuming that R15
is in use. Since Y will be used to input back into the scratchpad,
OEY = LOW and WE' will be driven by WRITE'/MSS'. The
complete microword for the Am2903 is:
The data flow is given in Figure 6-4.
Figure 6-4 Am2903: Increment a register and output its original
A 16-bit byte swap exchanges bits B15-B8,
with B7-B0 in order and visa versa.
Assume a 16-bit ALU (4 slices) and that the register is question
The Am2901 microcode solution can be constructed by taking advantage
of the fact that a shift up is equivalent to multiplying by 2. If
the result of an add of R15 + R15 is shifted
(prior to storing the result) into R15, the effect is
a 2-bit rotate, assuming that
- the high order input.output of the RAM shift is connected to
the low order input/output of the RAM
- the carryout Cn+4 is connected to the carry-in Cin.
Since we are selecting source operands as A, B, adding, and shifting,
2F --> B; the microword is:
The microinstruction must be repeated four times to cause an 8-bit
rotate, for a total timing of four microcycles of approximately
165 ns worst-case minimum (dependent onthe actual hardware).
[Remember that this was the late 1970s and early 1980s - times change.]
The Am2903 solution is approached in the same way. The S shift
is connected for rotate as was the RAM shift of the Am2901. The
carry out is input to the carry in, and an ADD is performed. The
complete microword is:
Am2901 Hardware Version
The byte-swap operation can be performed faster using
additional hardware (such as Am25LS240/244) tristate buffers (select
noninverting). The Am2901 Y outputs are gated out to the budders,
which in turn input to the appropriate DA inputs of another
Am2901. The operation performed by the Am2901 in this case is to
output from the scratchpad, Y -> A, and to pass the DA
input through the ALU and back into the scratchpad memory, F ->
B. The additional hardware allows the byte swap to be done in one
microcycle. The microword is:
Am2903 Hardware Version
There are two ways to perform a hardware-assisted byte swap using
the Am2903 and the Am25LS240/244 buffers.
One approach is to use the DB as the output to the buffers
and to use DA as the input point. This is the relatively
slower approach, since the DA input passes the data through
the ALU and back to the scratchpad.
The other approach uses the Y port as the data input, which provides
a datapath directly into the scratchpad. This method avoids the
propagation delay of the ALU, shift register, tristate buffer, and
input MUX. The microwords are:
The basic difference is the OEY control.