icemanxp300 wrote:
I swear this is like learning a foreign language
I'll try to define terms.
Quote:
ROM #1 /OE is enabled when SNES A21 is low and SNES ROM /OE is low.
ROM #2 /OE is enabled when SNES A21 is high and SNES ROM /OE is low.
I want to know what this low and high means. I am trying to understand the difference between a mad-1 and this 74LS00.
Low means the signal's voltage is close to ground (0 V). It represents a binary 0.
High means the signal's voltage is close to the supply voltage (+5V in the NES and Super NES). It represents a binary 1.
Active low means a signal is considered turned on when it is low. The name of an active low signal will begin with a slash, such as /OE.
/OE means an output enable that is active low. Chip enables on the NES and Super NES are active low more often than not.
/CE or
/CS is a chip enable (also called chip select) signal that is active low. Both /CE and /OE need to be low for a chip to send data to the CPU. The difference between a chip enable and an output enable is that when chip enable is high, the chip uses less power, but the chip reacts faster to output enable going low than to chip enable going low.
A
KiB (kibibyte) is 1024 bytes, or 2^10 bytes.
A
MiB (mebibyte) is 1048576 bytes, or 2^20 bytes.
LoROM describes a memory mapping in Super NES Game Paks that does not connect A15 on the cart edge to the ROM. This means each 64 KiB of address space contains only 32 KiB of data. The other half is used for input and output ports and RAM inside the system.
HiROM describes a memory mapping in Super NES Game Paks that connects A15 on the cart edge to the ROM. This means each 64 KiB of address space contains 64 KiB of data. The terms LoROM and HiROM are distantly related to the canonical state of A22 when accessing ROM.
When SNES A21 is low, that means it's trying to read the first 2 MiB of address space. When SNES A21 is high, that means it's trying to read the second 2 MiB of address space. (2^21 = 2097152, and 2097152 bytes is 2 MiB.) On LoROM, each 2 MiB of address space contains 1 MB of data. On HiROM, each 2 MiB of address space contains 2 MiB of data.