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62 lines
2.8 KiB
Plaintext
62 lines
2.8 KiB
Plaintext
// This file is part of www.nand2tetris.org
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// and the book "The Elements of Computing Systems"
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// by Nisan and Schocken, MIT Press.
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// File name: projects/5/Memory.hdl
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/**
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* The complete address space of the Hack computer's memory,
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* including RAM and memory-mapped I/O.
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* The chip facilitates read and write operations, as follows:
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* Read: out(t) = Memory[address(t)](t)
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* Write: if load(t-1) then Memory[address(t-1)](t) = in(t-1)
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* In words: the chip always outputs the value stored at the memory
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* location specified by address. If load=1, the in value is loaded
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* into the memory location specified by address. This value becomes
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* available through the out output from the next time step onward.
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* Address space rules:
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* Only the upper 16K+8K+1 words of the Memory chip are used.
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* Access to address>0x6000 is invalid. Access to any address in
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* the range 0x4000-0x5FFF results in accessing the screen memory
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* map. Access to address 0x6000 results in accessing the keyboard
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* memory map. The behavior in these addresses is described in the Screen
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* and Keyboard chip specifications given in the lectures and the book.
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*/
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CHIP Memory {
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IN in[16], load, address[15];
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OUT out[16];
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PARTS:
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// Memory = RAM16K + Screen + Keyboard (using address decoding)
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// step 1: decode high address bits to pick component
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// step 2: route load signal to correct component
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// step 3: select output from correct component
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//
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// addr map from c5 reading (for reference, duh):
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// 0x0000-0x3FFF: RAM16K (address[14]=0)
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// 0x4000-0x5FFF: Screen (address[14]=1, address[13]=0)
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// 0x6000: Keyboard (address[14]=1, address[13]=1)
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// address[14] choose RAM vs I/O
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// address[13] choose screen vs kb when I/O
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// STEP 1
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// split address to selectors
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Not(in=address[14], out=ramSel); // ramSel = 1 when address[14] = 0 (RAM range)
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And(a=address[14], b=address[13], out=kbSel); // kbSel = 1 when both bits = 1 (kb address)
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Not(in=address[13], out=notkb); // invert address[13] for screen selection
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And(a=address[14], b=notkb, out=screenSel); // screenSel = 1 when address[14]=1, address[13]=0
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// STEP 2
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// send load to correct component
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And(a=load, b=ramSel, out=ramLoad); // ramLoad = load when accessing RAM
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And(a=load, b=screenSel, out=screenLoad); // screenLoad = load when accessing screen
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// mem components
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RAM16K(in=in, load=ramLoad, address=address[0..13], out=ramOut); // 16K RAM uses 14 addr bits
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Screen(in=in, load=screenLoad, address=address[0..12], out=screenOut); // screen uses 13 addr bits
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Keyboard(out=kbOut); // kb is ro, no load/addr
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// STEP 3
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// pick correct out
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Mux16(a=screenOut, b=kbOut, sel=kbSel, out=ioOut); // pick screen or kb out
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Mux16(a=ramOut, b=ioOut, sel=address[14], out=out); // pick RAM or I/O out
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}
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