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Posted Thu Jan 17, 2019 4:59 PM
Posted Thu Jan 17, 2019 5:19 PM
How it works
The top 3 bits of the address are decoded into 8 enable lines by a 74LS138 decoder. Not all of these are needed, but the one that corresponds to A0,A1,A2 equal to 0,1,1 or addresses >6000 to >7fff is labeled ROMS* or cartridge select. This goes to the module port, where it is connected to a CE1* or chip enable pin on the cartridge ROM. The effect is that the cartridge ROM only 'wakes up' when one of those addresses is present on the address bus.
The rest of the address lines A3-A15 go into the ROM or RAM chip to select a byte.
In a similar way, the line responding to 0,0,0 goes to the CS* on and activates the console ROMs (16 bits wide). The line for 1,0,0 goes to yet another 74LS138 chip enable which decodes A3,A4,A5 to subdivide the >8000 - >9fff space into 8 blocks.
Posted Thu Jan 17, 2019 5:25 PM
Edited by GDMike, Thu Jan 17, 2019 5:25 PM.
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