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This question is posed because a Dell Optiplex 390 does not have AHCI. If it is a software feature then AHCI can be added. I am trying to follow the discussion associated with adding a kernel extension to enable a boot loader to "see" and format a SSD, despite lack of AHCI defined in the motherboard (BIOS?)

I am trying to reconcile that I have successfully outfitted the Dell with a SATA SSD and Ubuntu 16.04, whereas the BIOS test indicated there was no HDD. The test result led me to posting regarding OS X installs that indicated AHCI absence in the Optiplex 390.

Nothing is returned with:

dmesg | grep -i ahci

However,

dimes | grep -i scsi 

returns:

user@Brooklyn:~$ dmesg | grep -i scsi

[   0.134353] SCSI subsystem initialized

[   0.680830] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 249)

[   0.842793] scsi host0: ata_piix

[   0.842968] scsi host1: ata_piix

[   0.998803] scsi host2: ata_piix

[   0.998931] scsi host3: ata_piix

[   2.422963] scsi 0:0:0:0: Direct-Access   ATA     ADATA SU655     1b10 PQ: 0 ANSI: 5

[   2.423320] sd 0:0:0:0: Attached scsi generic sg0 type 0

[   2.425233] sd 0:0:0:0: [sda] Attached SCSI disk

[   2.448240] scsi 1:0:0:0: CD-ROM          PLDS    DVD-ROM DH-16D6S BD11 PQ: 0 ANSI: 5

[   2.508837] sr 1:0:0:0: [sr0] scsi3-mmc drive: 48x/48x cd/rw xa/form2 cdda tray

[   2.509019] sr 1:0:0:0: Attached scsi CD-ROM sr0

[   2.509187] sr 1:0:0:0: Attached scsi generic sg1 type 5

[  743.032182] scsi host4: usb-storage 2-1.3:1.0

[  744.031462] scsi 4:0:0:0: Direct-Access  SanDisk  Cruzer Glide   1.00 PQ: 0 ANSI: 6

[  744.032003] sd 4:0:0:0: Attached scsi generic sg2 type 0

[  744.064357] sd 4:0:0:0: [sdb] Attached SCSI removable disk

Not sure what you mean by "feature".

AHCI is the Advanced Host Controller Interface. In other words, it's standard for SATA controllers (a piece of hardware) and describes how the registers are layout so that the driver (a piece of software) can access them.

For legacy reasons, some IDE/SATA controllers (a piece of hardware) have multiple ways to lay out their registers, and AHCI is one way among them. Then the BIOS (a piece of firmware) can configure the controller register layout (by writing to a special register).

I'll leave it to you if this is a hardware "feature", a software "feature", a firmware "feature", or all of it.

In any case, your Dell possibly has some kind of disk controller. (Or possibly it doesn't even have SATA, it might have NVRAM storage instead). The disk controller, if it is there, may support an AHCI register layout, or it may not; only owners of this Dell model can find out.

But as long as you can access your harddisk (or NVRAM "disk") because you have the necessary drivers, you needn't worry about that. And you don't need to "add this feature".

Now if your BIOS has a way to switch the SATA controller to AHCI, you are encouraged to do so, because it's a better way to access the controller than the legacy methods. But that's all there is to it.