The Finnish member largon of the English-speaking Xtremesystems-forum
has made an interesting discovery by having a look at the RV770-GPU. To be exact he looked at the SIMD-cores of the chip on a schemata-subscription and came to a surprising conclusion.
On the RV770 there are 180 instead of 160 vec5-units synonymic with 900 instead of 800 streamprocessors. But one by one:
The current picture shows the SIMD-cores of the RV770. The row by row to ten numbered figures correspond to the SIMD-units of AMD-GPU. The column by column to nine numbered figures the shader clusters per SIMD, in which two shader units are located. Consequently, it has 18 shader cores per SIMD-unit.
The product of both numbers is the count of vec5-units, so 180 (10*18). Because every vec5-unit – as the name says – has five different scalar units (RGBA, red green, blue, alpha stock, special function) you can speak of 900 stream processors.
But what does our schemata-subscription tell us? In reality the RV770 has only 800 stream processors from ten SIMD-units and 16 instead of 18 shader cores per SIMD.
We can speculate that AMD could have obstructed two extra shader cores per SIMD only to increase the yieldrate of the chip. Are one or two shader cores per SIMD-unit broken the chip must not be directly trashed.
Underhand you could also wonder if AMD could activate the two disabled shader units per SIMD in a prospective video card. 900 instead of 800 streamprocessors would be synonymic with a 12.5 per cent higher shader performance on the paper.
But this is more than improbable: The shader cores have to be always a multiple of four because quads – groups of four pixels – must be calculated. The extra shaders could be used as coprocessors to calculate GPGPU-applications.
An interesting detail, that finds first six months after the official presentation the why outwards.