Hardware Reference
In-Depth Information
looks perfectly reasonable, but there is no way to execute it on the data path of
Fig. 4-6 in one cycle. This restriction exists because for an addition (other than
increment or decrement) one of the operands must be the
H
register. Likewise,
H=H
−
MDR
might be useful, but it, too, is impossible, because the only possible source of a
subtrahend (the value being subtracted) is the
H
register. It is up to the assembler
to reject statements that look valid but are, in fact, illegal.
DEST = H
DEST =
S
OURCE
DEST =
H
DEST = SOURCE
DEST=H+SOURCE
DEST=H+SOURCE + 1
DEST=H+1
DEST = SOURCE + 1
DEST = SOURCE
H
DEST = SOURCE
−
1
DEST =
−
H
DEST = H AND SOURCE
DEST=HORSOURCE
DEST = 0
DEST = 1
DEST =
−
−
1
Figure 4-16.
All permitted operations. Any of the above operations may be ex-
tended by adding ''<< 8'' to them to shift the result left by 1 byte. For example, a
common operation is
H
=
MBR
<<8.
We extend the notation to permit multiple assignments by the use of multiple
equal signs. For example, adding 1 to
SP
and storing it back into
SP
as well as
writing it into
MDR
can be accomplished by
SP=MDR=SP+1
To indicate memory reads and writes of 4-byte data words, we will just put
rd
and
wr
in the microinstruction. Fetching a byte through the 1-byte port is indicated
by
fetch
. Assignments and memory operations can occur in the same cycle. This
is indicated by writing them on the same line.
To avoid any confusion, let us repeat that the Mic-1 has two ways of accessing
memory. Reads and writes of 4-byte data words use
MAR
/
MDR
and are indicated in
the microinstructions by
rd
and
wr
, respectively. Reads of 1-byte opcodes from the
