early DSP

[Chuck Guzis]

On 1 Jan 2007 at 10:47, dwight elvey wrote:

> OK, I'll bite. What were the two instructions? Most DSP's
> are built around a MAC. I would guess the large word length
> might be to include immediate coefficients.

Well, the 7720 has 3 or 4 instructions according to the manual (I 
don't know who wrote it, but it's very confusing) which seems not to 
be able to make up its mind.

Storage is a 23-bit instruction ROM, a 16-bit data ROM and a 16-bit 
data RAM.  Architecture is basically 1-address, with two selectable 
accumulators.

Instruction I/II, OP and RT:  Same format, but the RT also performs a 
subroutine return at the conclusion.  Fields designate ALU function, 
source, destination, and address register modification.  Instruction 
is single-cycle.

Instruction III, JP:  Jump; does conditional, unconditional jumps and 
subroutine call.

Instruction IV,  LD:   Immediate load of value to selected 
destination.

Now, I'd say that saying that there are only 3 instructions is sort 
of cheating, but then, I didn't write the manual.

NEC doesn't call this chip a DSP; but an SPI (signal processing 
interface).  But it has the DSP earmark of compute-and-accumulate 
capability with a 16x16 single-cycle multiplier.

Literature seems to date the chip sometime in 1983.  Unfortunately, I 
don't have one of the chips.

Cheers,
Chuck