Texas Instruments 990 Operation Manual page 158

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945255-9701
Assume that the beginning memory word is ati6cation 0100
16 •
The displacement in loop level 2 is 0;
therefore, the first beginning address to be used by loop level 1 is 0100
16 •
Since loop level 1 has a
displacement of 0, the first bit string has a beginning address of bit 0 of address 0100
16 •
Proceeding by
adding the bit increment of 4 to each bit address, the next three bit strings can be selected. Loop level 1
has now been completed. Going back to loop level 2 and the previous beginning address in memory (bit
o
of address 0100
16 ),
add the bit incremeht of 10
16
to that address. The new beginning address in
memory is bit 0 of address 0102
16 ,
which is now used by loop level! to select the next four bits strings.
When those strings have been selected, loop level 2 then determines the third beginning address in
memory by adding the bit increment 10
16
to the previous address of bit 0 of address 0102
16 .
Selecting
a new beginning address and using that address to increment through loop levell, loop level 2
continues until 20
16
beginning addresses have been selected and loop level 1 has been processed 20
16
times.
7.4.5.3 Three-Level Looping. Loop level 3 can be used for reiterative programming. Assume the user
wishes to program a 256 by 4 PROM using the same memory data configuration as in the previous
example. Since the previous example only selects 128 four-bit strings, the first 128 words and the last
128 words of the PROM could be programmed with the same data from memory.
The PROM data configuration is standard and the control information can be read from the Standard
Control Information Cassette. The memory data configuration would have the following parameters:
Loop level
Bit increments (I)
Number of iterations (1)
Initial bit displacement (1)
Bit string width
Loop level
Bit increment (2)
Number of iterations (2)
Initial bit displacement (2)
Bit string width
Loop level
Bit increment (3)
Number of iterations
(3)
Initial bit displacement (3)
Bit string width
=
I
=4
=4
=0
=4
=2
=
10
16
=
20
16
=0
=4
=3
=0
=2
=0
=4
Assuming the beginning memory address is 0100
16 ,
the loop level 3 displacement 0 is added to that
address to get the beginning address for loop level 2. The increment described for two-level looping
now is performed. When the incrementing is complete (128 bit strings of four bits have been selected),
loop level 3 then determines the next beginning address for loop level 2. Since the bit increment for
loop level 3 is 0, the second beginning address is the same as the first one. Therefore the two-level
looping increments through the same memory configuration and selects the same 128 bit strings to
program the second 128 four-bit words of the PROM.
The standard PROM control information for a 256 by 4 PROM includes the parameters:
Loop level
Bit increment (I)
Number of iterations (1)
Initial bit displacement (1)
Bit string width
=
1
=4
=256(100
16 )
=0
=4
7-8
Digital Systems Division .