Veja a resposta do @ bahamat a esta pergunta intitulada: Opções de parada e desligamento para o comando shutdown .
.... halt was used before ACPI (which today will turn off the power for you)*. It would halt the system and then print a message to the effect of "it's ok to power off now". Back then there were physical on/off switches, rather than the combo ACPI controlled power button of modern computers. ...
O comando halt também faria outras coisas para colocar o sistema em um bom estado conhecido antes de parar, como sincronizar os discos e enviar os comandos para a CPU para que ela soubesse como HALT.
Veja a página da Wikipedia sobre o HCF - Halt and Catch Fire .
trecho
Halt and Catch Fire, known by the mnemonic HCF, refers to several computer machine code instructions that cause the CPU to cease meaningful operation. The expression "catch fire" is intended as a joke; the CPU does not catch fire. It is also occasionally referred to as "SDI" for "Self Destruct Immediate".