4 ms·
Did the video explain why poking one value, N, resulted in a peek of the value 240+N? Did I miss it? Is it just a property of the memory location on the VIC c
by rrauenza 10y ago
Did the video explain why poking one value, N, resulted in a peek of the value 240+N? Did I miss it? Is it just a property of the memory location on the VIC chip?
- lefticus 10y agoIt did not explain why, I'm assuming it's a property of the VIC, but I haven't looked into it yet.
- lefticus 10y agoHere, the are spec'd as unused bits returning 1: http://www.antimon.org/code/vic2.html http://www.antimon.org/code/vic2.html
- rrauenza 10y agoAh, thank you. It appears it is only a 4 bit memory location and the spec says the unused bits always return 1's.
- white-flame 10y agoThe NMOS logic circuitry of the time had pull-up resistors bringing up the voltages to 5V. Chips would then ground the lines to make 0V. So in many logical views, '1' is the default state, and '0' is the active state. Unconnected bits would often leave the lines at 1. So, if you ask a chip to read a value, and it asserts 4-bit number on the 8-bit data bus, it would tend to be 1111xxxx in binary, or 240+x. (Note: I don't know if it asserts 4 bits on the bus, and the bus pull-ups keep the upper 4 bits high, or if it's internal to the chip that the bits are naturally pulled up and it in fact asserts all 8 bits. Same net effect, modulo some sub-cycle rise/fall timing.)