Apparently IBM XT also had a way of being bootstrapped with no drives attached

https://hackaday.com/2011/10/03/resurrecting-an-xt/

    ;————————————————————————
    ; LOAD A BLOCK OF TEST CODE THROUGH THE KEYBOARD PORT :
    ; FOR MANUFACTURING TEST. :
    ; THIS ROUTINE WILL LOAD A TEST (MAX LENGTH=FAFFH) THROUGH :
    ; THE KEYBOARD PORT. CODE WILL BE LOADED AT LOCATION :
    ; 0000:0500. AFTER LOADING, CONTROL WILL BE TRANSFERRED :
    ; TO LOCATION 0000:0500. STACK WILL BE LOCATED JUST BELOW :
    ; THE TEST CODE. THIS ROUTINE ASSUMES THAT THE FIRST 2 :
    ; BYTES TRANSFERRED CONTAIN THE COUNT OF BYTES TO BE LOADED :
    ; (BYTE 1=COUNT LOW, BYTE 2=COUNT HI.) :
    ;————————————————————————