O código-fonte do hdparm mostra que apenas passa o valor para o disco, exceto que passa o comando 0x85 em vez de 0x05 quando o valor é 255. O ATA Spec mostra este detalhe:
Subcommand code 05h allows the host to enable Advanced Power Management. To enable Advanced Power Management, the host writes the Sector Count register with the desired advanced power management level and then executes a SET FEATURES command with subcommand code 05h. The power management level is a scale from the lowest power consumption setting of 01h to the maximum performance level of FEh. Table 30 shows these values.
- Desempenho máximo FEh
- Níveis intermediários de gerenciamento de energia sem espera 81h-FDh
- Consumo mínimo de energia sem 80 horas de espera
- Níveis intermediários de gerenciamento de energia com Standby 02h-7Fh
- Consumo mínimo de energia com 01h de espera
- FFh reservado
- Reserved 00h
Device performance may increase with increasing power management levels. Device power consumption may increase with increasing power management levels. The power management levels may contain discrete bands. For example, a device may implement one power management method from 80h to A0h and a higher performance, higher power consumption method from level A1h to FEh. Advanced power management levels 80h and higher do not permit the device to spin down to save power. Subcommand code 85h disables Advanced Power Management. Subcommand 85h may not be implemented on all devices that implement SET FEATURES subcommand 05h.
Em outras palavras: "Não especificado, comportamento específico do dispositivo"
