The microprocessor characteristic which possibly most permits the computing knowledge we consider for granted today is protected method. A chip with the needed hardware can run specific application procedures in their individual environments, enabling multitasking and isolation among processes. More mature CPUs lacked this attribute, which means that all the resources ended up accessible to all software. [Andy Hu] has finished the seemingly unattainable with a Zilog Z80, enabling a guarded manner on the chip for the initial time in about 4 decades. Has he located an elusive undocumented piece of silicon missed by every other researcher? Not quite, but it is a intelligent hack.
The Z80 has two address spaces, one particular for memory and the other for I/O. He’s taken the I/O request line and fed it by a flip-flop and some logic to call a components interrupt the initial time an I/O connect with is created or when a RST instruction is executed. Coupled with a compact piece of memory for register contents, and he’s built a Z80 with a totally-purposeful protected mode, for the expense of a couple logic chips. It is spelled out in the video clip below the break, and we hope you concur that it’s fairly stylish presented the assets in hand. It’s much too late for the business 8-little bit devices of the previous, but it would be intriguing to see what today’s retrocomputer designers make of it.