The Apple-I was an unlike Apple’s later items. A $666 single-board computer system, the item had some special layout functions consisting of utilizing a change register for video clip memory to conserve cash. The change signs up of the day called for high-current clock pulses that varied from -11 to 5V and also there was a DS0025 clock vehicle driver chip to take care of the work. [Ken Shirriff] takes the unusual chip apart for us in a current post.
Using a change register as memory isn’t an originality. Truly old computer systems like EDSAC utilized mercury hold-up lines as memory which was basically a physical change register. In those instances, the ALU and also various other handling just needed to manage a little bit at once, additional streamlining points. For the Apple, there were 7 change signs up to save 6-bits of display screen information and also an arrow placement. The 6 littles personality information drove– indirectly– a personality generator ROM to transform the information right into dots for the display screen.
Driving all those change register flip flops needs a great deal of clock present, so the DS0025 utilizes an uncommon transistor layout. There are 24 different emitters in 2 teams. It imitates a big transistor, yet you can additionally consider it as 2 12-emitter transistors or 24 different transistors in parallel. The steel electrical wiring, remarkably sufficient, tapers since at the beginning of the conductor, the present for all 12 sub-transistors moves, yet by the end, it is just the present for the last sub-transistor, so the conductor does not need to be as vast. Additionally, both transistors need to have actually matched resistance which needs cautious layout so the transistors activate at the very same time.
The result is an inverter that can give 1.5 amps. This present assists get over the fairly big capacitance in the change register’s clock line. The clock price was 1 MHz and also the lots capacitance had to do with 150 picofarads.
We appreciate [Ken’s] messages varying from enigmas to room equipment. It is constantly fascinating to see what is inside these tools or, at the very least, what remained in the old tools we have actually all seen.