IA2910A
Data Sheet
Microprogram Controller
August 19, 2008
Instruction 4 PUSH is the PUSH/CONDITIONAL LOAD COUNTER instruction and is used
primarily for setting up loops in microprogram firmware. In Figure II, when instruction 52 is in the
pipeline register, a PUSH will be made onto the stack and the counter will be loaded based on the
condition. When a PUSH occurs, the value pushed is always the next sequential instruction address.
In this case, the address is 53. If the test fails, the counter is not loaded; if it is passed, the counter
is loaded with the value contained in the pipeline register branch address field. Thus, a single
microinstruction can be used to set up a loop to be executed a specific number of times. Instruction
8 will describe how to use the pushed value and the register/counter for looping.
Instruction 5 JSRP is a CONDITIONAL JUMP-TO-SUBROUTINE via the register/counter or
the contents of the PIPELINE register. As shown in Figure II, a PUSH is always performed and
one of two subroutines executed. In this example, either the subroutine beginning at address 80 or
the subroutine beginning at address 90 will be performed. A return-from-subroutine (instruction 10)
returns the microprogram flow to address 55. In order for this microinstruction control sequence to
operate correctly, both the next address fields of instruction 53 and 54 would have to contain the
proper value. Let’s assume that the branch address fields of instruction 53 contain the value 90 so
that it will be in the IA2910A register/counter when the contents of address 54 are in the pipeline
register. This requires that the instruction at address 53 load the register/counter. Now, during the
execution of instruction 5 (at address 54), if the test failed, the contents of the register (value = 90)
will select the address of the next microinstruction. If the test input passes, the pipeline register
contents (value = 80) will determine the address of the next microinstruction. Therefore, this
instruction provides the ability to select one of two subroutines to be executed based on a test
condition.
Instruction 6 CJV is a CONDITIONAL JUMP VECTOR instruction which provides the
capability to take the branch address from a third source heretofore not discussed. In order for this
instruction to be useful, the IA2910A output, VECTn is used to control a three-state control input
of a register, buffer, or PROM containing the next microprogram address. This instruction
provides one technique for performing interrupt type branching at the microprogram level. Since
this instruction is conditional, a pass causes the next address to be taken from the vector source,
while failure causes the next address to be taken from the microprogram counter. In the example of
Figure II, if the CJV instruction is contained at location 52, execution will continue at vector address
20 if the CCn input is LOW and the microinstruction at address 53 will be executed if the CCn input
is HIGH.
Instruction 7 JRP is a CONDITIONAL JUMP via the contents of the IA2910A register/counter
or the contents of the pipeline register. This instruction is very similar to instruction 5; the
conditional jump-to-subroutine via R or PL. The major difference between instruction 5 and
instruction 7 is that no push onto the stack is performed with 7. Figure II depicts this instruction as
a branch to one of two locations depending on the test condition. the example assumes the pipeline
register contains the value 70 when the contents of address 52 is being executed. As the contents of
address 53 is clocked into the pipeline register, the value 70 is loaded into the register/counter in the
IA2910A. The value 80 is available when the contents of address 53 is in the pipeline register.
Thus, control is transferred to either address 70 or address 80 depending on the test condition.
Instruction 8 RFCT is the REPEAT LOOP, COUNTER NOT EQUAL ZERO instruction. This
microinstruction makes use of the decrementing capability of the register/counter. To be useful,
some previous instruction, such as 4, must have loaded a count value into the register/counter. This
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