| ??? 12/23/08 11:14 Read: times |
#161114 - Overhead Responding to: ???'s previous message |
Ahmed Lazreg said:
If we access them directly with Rxx, it's quicker than access to ram location directly only if we are in the good bank. If we need bank switching, it will be slower (one more operation). If you did only one access to one register, that may well be true. But, in general, you switch the bank and then do many accesses in that bank - so the overhead of the bank switch is greatly offset by the savings in all the subsequent Rn accesses. This is precisely why there are multiple addressing modes - because no single mode is ideal for every different situation. As always in Enigneering, it's a tradeoff: everything has its costs and benefits - it's up to the Engineer to work out which gives the best compromise in each particular situation... Are there any instructions that does work only with the Rxx notation and not with direct addressing ? Yes, there are - you can check them out in the Instruction Set document - which is Chapter 2 of the so-called "bible" for the 8051: Chapter 1 - 80C51 Family Architecture: http://www.nxp.com/acrobat_d...ARCH_1.pdf Chapter 2 - 80C51 Family Programmer's Guide and Instruction Set: http://www.nxp.com/acrobat_d...UIDE_1.pdf Chapter 3 - 80C51 Family Hardware Description: http://www.nxp.com/acrobat_d...WARE_1.pdf |
| Topic | Author | Date |
| What is banked access needed for ? | 01/01/70 00:00 | |
| Register Banks? | 01/01/70 00:00 | |
| 3 bits in single instruction byte | 01/01/70 00:00 | |
| and bank switching ? | 01/01/70 00:00 | |
| Overhead | 01/01/70 00:00 | |
Got it now | 01/01/70 00:00 |