ST20-C1 datasheet(57/205 Pages) STMICROELECTRONICS

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5 Multiply accumulate

The data items from each vector are read from memory in turn and the products

formed between corresponding pairs from the two vectors. Each of these products is

added into the running accumulator value. The instruction completes with 3 values in

the stack - the final accum ulator value and the two updated data pointers.

Four control values are held in the status register, as shown in Table 5.2.

These values are initialized by the

smacinit instruction. The smacinit instruction takes

a packed control word in Areg, extracts the control fields and loads these into the

status register. For

smacinit, Areg is organized as shown in Table 5.3.

These status register values are global and are not saved when a process is times-

liced or descheduled. If more than one process is performing short multiply accumu-

late loops then the values should be reloaded by the process code using

smacinit

after each

timeslice and stop instruction.

5.3.1

X buffer size

The X vector buffer size is determined by the mac_buffer control field, which may take

the values 0 to 7. When mac_buffer is 0, then no address wrapping takes place, i.e.

the buffer is assumed to be of infinite size. Otherwise, the buffer size is 2mac_buffer+2,

as shown in Table 5.3. The X buffer must be aligned to a multiple of its own size, so a

buffer of N bytes must start at an address whose value is a multiple of N bytes.

5.3.2

Number of steps

The mac_count control field in the status register deter mines the number of multiply-

accumulate steps for

smacloop. This is an unsigned 8-bit integer. The value zero

signifies 256 steps; otherwise the v alue of mac_count is the number of steps.

Field

Size

Meaning

mac_count

8 bits The number of steps (from 1 to 256 items).

mac_buffer

3 bits The size code for the data buffer within which the data vector lies.

mac_scale

2 bits Shift control for scaling coefficient v alues.

mac_mode

1 bit

Accumulator format - 0 indicates 16-bit (short) and 1 indicates 32-bit (long) value.

Table 5.2

smacloop status register fields

Field

Size

Least significant

bit

Most significant

bit

mac_count

8 bits

mac_buffer

3 bits

mac_scale

2 bits

mac_mode

1 bit

Table 5.3

smacinit Areg format

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