Examples Using One, Two, And Three-Level Looping - Texas Instruments 990 Operation Manual

J2n.7\. _ _ _ _ _ _ _
'-i:Y
945255-9701
The PROM/ROM bounds are set to 0 and FF
16,
giving a string of 256 X 4
=
1024 bits. The memory
bounds are set to 0 and IFF
16,
giving a memory string of256 X 16
=
4096 bits. The bit string width is
set to 4, which is equal to the PROM word width. The PROM/ROM mapping parameters are the
following:
Initial bit displacement
Bit increment
Number of iterations
=0
=4
=
100
16
The initial bit displacement is 0 so that the string will be transferred to the PROM starting at bit 0 of the
PROM word. The bit increment is
4
so that each string of
4
bits will be stored consecutively in the
PROM. The number of iterations is 100
16
(256
10 )
so that 256 bit strings of
4
bits each will be
transferred.
The memory mapping parameters for PROM I are the following:
Initial bit displacement
Bit increment
Number of iterations
=0
=
10
16
=
100
16
The initial bit displacement selects the bit string starting at bit 0 of the memory word. The bit
increment is 10
16
(161()) so that each successive bit string will begin at bit 0 of each l6-bit memory
word. The number of iterations is 100
16
(256
10 )
so that 256 memory bit strings of 4 bits each will be
transferred.
Initial bit displacements of 4
16 ,
8
16 ,
and C
16
are used to transfer the remaining bits of the memory
words to PROMs II, III, and IV, respectively. These bit displacements will select the 4 bit strings
beginning at bits 4
16 ,
8
16 ,
and C
16
of each 16-bit memory word. .
The mapping parameters in this example define the level one looping. Level two and three looping are
not used in this example since the pattern defined by the level one looping is not repeated.
7.4.5 EXAMPLES USING ONE, TWO, AND THREE-LEVEL LOOPING. The following examples
take the user through the PROM programming process step by step using various levels of looping.
7.4.5.1 One-Level Looping. An address is determined in the following manner. Starting with the
beginning memory/PROM/ROM· address (indicated by the MB or RB subcommand, paragraph 7.5.3.1
or 7.5.3.2), the initial bit displacement for loop level 1 is added to that address. The resulting bit
address is the beginning address of the first bit string. To get each consecutive bit string beginning
address, the number of bits indicated by the bit increment value for loop level 1 is added to the previous
address. This process continues until the number of addresses determined has reached the maximum
value for loop level 1.
As an example, assume a user wishes to select the first four bits of each word of a 256-word block of
data in memory. The parameters needed would be:
Loop level
Bit increment
(1)
Number of iterations (1)
Initial bit displacement (I)
Bit string width
=
I
=
16 (10
16 )
=
256 (100
16 )
=0
=4
7-6
Digital Systems Division