The Radeon HD 5550 Architecture
We’ll start by looking at a block diagram of the new Radeon HD 5550. The GPU is actually the same RV830-based silicon used in the Radeon HD 5570/5670, but with some functionality disabled.
In the big scheme of things, not much is changed. The only notable modification is that the Radeon HD 5550 GPU has four single-instruction multiple-data (SIMD) engines, while the Radeon HD 5570/5670 has five. Each SIMD engine features four texture units and 16 stream processors, and each stream processor has five ALUs (AMD calls them stream cores). Therefore, the Radeon HD 5550 GPU has 320 stream cores and 16 texture units, compared to the Radeon HD 557/5670’s 400 stream cores and 20 texture units. Aside from this 20% reduction, nothing else has been disabled. The Radeon HD 5550 sports the same dual 64-bit memory controllers sharing two render back-ends as the Radeon HD 5570/5670. Each render back-end contains four color render output units (ROPs) resulting in a total of eight ROPs and a combined 128-bit memory interface.
The end result is that the Radeon HD 5550 GPU is quite powerful for a low-end part (certainly a lot more powerful than the Radeon HD 5450 and its 80 stream cores). Often, when low-end GPUs are designed, the render back-ends are sacrificed. But the Radeon HD 5550 maintains the same ROP count as the Radeon HD 5570 and 5670. To get the whole story though, let’s look at the complete list of specifications:
| Header Cell - Column 0 | Radeon HD 5550 | Radeon HD 5570 | Radeon HD 4670 |
|---|---|---|---|
| Shader Processors: | 320 | 400 | 320 |
| Texture Units: | 16 | 20 | 32 |
| Color ROPs: | 8 | 8 | 8 |
| Fabrication process: | 40 nm | 40 nm | 55 nm |
| Core Clock: | 650 MHz | 650 MHz | 750 MHz |
| Memory Clock: | 900 MHz DDR3900-1000 MHz GDDR5 | 900 MHz DDR3900-1000 MHz GDDR5 | 1000 MHz DDR3 |
| Memory Bus: | 128-bit | 128-bit | 128-bit |
| Data Rate: | 1.8 Gb/s DDR33.6-4 Gb/s GDDR5 | 1.8 Gb/s DDR33.6-4 Gb/s GDDR5 | 2 Gb/s DDR3 |
| Compute (GFLOPs): | 352 | 520 | 480 |
| Transistors (Millions): | 627 | 627 | 514 |
| Max Power (W) | 39 | 42.7 | 59 |
| Idle Power (W) | 10 | 10 | 14 |
The numbers show us how AMD is wedging the Radeon HD 5550 into the 5000-series: by dropping the GPU's core clock in order to keep performance in line with the product’s placement. But overclockers can see the Radeon HD 5550’s potential. The RV830 GPU should be able to handle a 700 MHz operating frequency with ease, and at that speed, the Radeon HD 5550 should be quite competitive with the Radeon HD 5570.
On top of this, the GDDR5 version of the Radeon HD 5550 offers more than twice the memory bandwidth of AMD's Radeon HD 5570 DDR3. It’s quite possible that could offset the core clock (and possibly the 80-core advantage) of the Radeon HD 5570. If this proves to be the case in the benchmarks, budget-oriented gamers will have an attractive board to tweak and tune. Of course, it'd be a little awkward for AMD, with the Radeon HD 5550 DDR5 and Radeon HD 5570 DDR3 performing uncomfortably similarly. We’ll keep our eyes on these two cards in the upcoming test results.
Finally, let's take a look at the new GDDR5-based Radeon HD 5570 and see how it stacks up against the Radeon HD 5670 and Radeon HD 5570 DDR3:
| Header Cell - Column 0 | Radeon HD 5670 | Radeon HD 5570 GDDR5 | Radeon HD 5570 DDR3 |
|---|---|---|---|
| Shader Processors: | 400 | 400 | 400 |
| Texture Units: | 20 | 20 | 20 |
| Color ROPs: | 8 | 8 | 8 |
| Core Clock: | 775 MHz | 650 MHz | 650 MHz |
| Memory Clock: | 1000 MHz GDDR5 | 900-1000 MHz GDDR5 | 900 MHz DDR3 |
| Memory Bus: | 128-bit | 128-bit | 128-bit |
| Data Rate: | 4 Gb/s GDDR5 | 3.6-4 Gb/s GDDR5 | 1.8 Gb/s DDR3 |
| Compute (GFLOPs): | 620 | 520 | 520 |
| Transistors (Millions): | 627 | 627 | 627 |
| Max Power (W) | 61 | 42.7 | 42.7 |
| Idle Power (W) | 14 | 10 | 10 |
Armed with the same GDDR5 memory as the next-highest model in AMD's Radeon HD 5000 family, the only functional difference between the Radeon HD 5570 and 5670 is 125 MHz of core clock speed. The Radeon HD 5570 GDDR5 might reach Radeon HD 5670 performance when overclocked, but 775 MHz on the core might be a bit of a stretch without a voltage increase. We’ll see how things turn out when we try our hand at overclocking these cards.