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| 04:15.50 | bsdfox | how does two's complement work in a register pair? do both registers have a sign bit on the MSB or just the high bit? what about registers that are only partially used (ie 8 bit high register with only 2 data bits) |
| 04:17.09 | MonMotha | bsdfox: the uppermost used bit is always the sign bit. The rest of the register is generally left untouched on partial-word operations, but some architectures will automatically sign extend. There's usually a sign extend instruction if it's not done automatically. |
| 04:17.49 | bsdfox | ok so on a pair we'd only have one sign bit and it would be directly above the MSB data bit |
| 04:17.50 | MonMotha | if there's a sign flag, it's updated based on the sign resulting from the instruction (which would be the MSB of the operated data, not necessarily the whole word) |
| 04:17.57 | MonMotha | are you referring to x86? |
| 04:18.00 | MonMotha | like al and ah |
| 04:18.07 | bsdfox | no it's for an accelerometer |
| 04:18.16 | MonMotha | oh, then just look in the datasheet |
| 04:18.33 | MonMotha | you mean there's e.g. two 8-bit registers that together are a 16-bit quantity? |
| 04:18.39 | MonMotha | there will generally only be one sign bit in that case, in the MSB |
| 04:18.56 | bsdfox | two 8-bit registers that are a 10-bit quantity |
| 04:19.10 | MonMotha | then the sign bit is PROBABLY the 3rd bit in the MSB |
| 04:19.12 | bsdfox | would it be bit7 in the high register or bit3? |
| 04:19.18 | MonMotha | the quantity may or may not be sign extended |
| 04:19.36 | MonMotha | depends on how they made it |
| 04:19.38 | bsdfox | all the datasheet says is to check the app note |
| 04:19.50 | bsdfox | and the app note just says it follows two's complement conventions |
| 04:19.53 | MonMotha | the safest thing to do would be to sign extend it yourself |
| 04:20.16 | MonMotha | LSB has the least significant 8 bits, MSB has two more data bits (bits 0 and 1), then bit 2 has the sign |
| 04:20.26 | MonMotha | treat bits 3-7 as undefined |
| 04:20.35 | MonMotha | they may be copies of the sign bit, or they may be fixed, or they may be random |
| 04:20.48 | bsdfox | ok that's a good idea |
| 04:20.58 | bsdfox | I'll just write the sign bit to bit2-7 |
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| 04:21.18 | MonMotha | the sign bit probably IS bit 2 |
| 04:21.24 | MonMotha | unless they have it in a separate register, which would be odd |
| 04:24.32 | bsdfox | http://cache.freescale.com/files/sensors/doc/app_note/AN3745.pdf is the app note. I don't see where it says anything concrete but I haven't tried just signing the high byte yet |
| 04:55.12 | bsdfox | oh god it actually does show that bit2 in MSB is the polarity bit |
| 04:55.31 | bsdfox | err the datasheet does. thanks for pointing me in the right direction |
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