IA2910A
Data Sheet
Microprogram Controller
August 19, 2008
Theory of Operation
Table 2 shows the result of each instruction in controlling the mux which determines the YOUT
outputs, and in controlling the three enable signals PLn, MAPn, and VECTn. The effect on the
register/counter and the stack after the next positive-going clock edge is also shown. The mux
determines which internal source drives the YOUT outputs. The value loaded into the counter is
either identical to the YOUT output, or else one greater, as determined by CIn. For each
instruction, one and only one of the three outputs PLn, MAPn, and VECTn is LOW. If these
outputs control three-state enables for the primary source of microprogram jumps (usually part of a
pipeline register), a PROM which maps the instruction to a microinstruction starting location, and
an optional third source (often a vector from a DMA or interrupt source), respectively, the three-
state sources can drive the DIN inputs without further logic.
Several inputs, as shown in Table 2, can modify instruction execution. The combination CCn
HIGH and CCENn LOW is used as a test in 9 of the 16 instructions. RLDn, when LOW, causes
the DIN input to be loaded into the register/counter, overriding any HOLD or DEC operation
specified in the instruction. OEn, normally LOW, may be forced HIGH to remove the IA2910A
YOUT outputs from a three-state bus.
The stack, a nine-word last-in, first-out 12-bit memory, has a pointer which addresses the value
presently on the top of the stack. Explicit control of the stack pointer occurs during instruction 0
(RESET), which makes the stack empty by resetting the SP to zero. After a RESET, and whenever
else the stack is empty, the contents of the top of stack is undefined until a PUSH occurs. Any
POPs performed while the stack is empty put undefined data on the F outputs and leave the stack
pointer at zero.
Any time the stack is full (nine more PUSHes than POPs have occurred since the stack was last
empty), the FULLn warning output occurs. This signal first appears on the microcycle after a ninth
PUSH. No additional PUSH should be attempted onto a full stack; if tried, information within the
stack will be overwritten and lost.
IA211030314-03
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