Nand2Tetris Hack Divide By 2 (Right Shift)

// Divides R0 by 2 and stores the dividend in R1 and remainder in R2
// (R0, R1, R2 refer to RAM[0], RAM[1], and RAM[2], respectively.)

// how : counter goes from 2 to 32768 ( doubling each time )
// start with 1 and dump result of the AND in R2
// start off setting R1 to 0 and after that, based on result of the AND of
// counter with R0, you either OR R1 with counter or do nothing

@R1
M = 0
D = 1
@R0
D = M & D
@R2
M = D // remainder captured
@lagcount
M = 1
@2
D = A
@counter
M = D

(LOOP)
@R0
D = M & D // we're expecting D (pre) to already have the counter value based on knowing our code
@NO_ACTION
D, JEQ
@lagcount // else, we need to OR R1 with the lagging counter
D = M
@R1
M = M | D
(NO_ACTION)
@16384 // can only load a 15 bit number :)
D = A
D = D + A
@counter
D = M - D
@END
D, JEQ
// else, we need to double counter and double lagcount
@lagcount
D = M
M = M + D
@counter
D = M
MD = M + D

@LOOP
0, JMP

(END)
@END
0, JMP

Shimon : Once Again I Have to Remind You..

that, if you don't understand this code, you have to stop and convince yourself that you understand it.

Make sure that you understand this intricate business of indirect addressing using the A register.

Nand2Tetris Hack Assembly Language Divide

// divide.asm
// Divides R0 by R1 and stores the dividend in R2 and remainder in R3
// (R0, R1, R2 refer to RAM[0], RAM[1], and RAM[2], respectively.)

@R2
M = 0
@R3
M = 0
@R0
D = M
@END
D, JEQ
@store
M = D // store to restore
(LOOP)
@R1
D = D - M
@REMAINDER
D, JLT
@R2
M = M + 1
@EVENLY
D, JEQ
@LOOP
0, JMP

(REMAINDER)
@R1
D = D + M
@R3
M = D
(EVENLY)
@store
D = M
@R0
M = D

(END)
@END
0, JMP

Nand2Tetris Screen Addressing

http://www.nand2tetris.org/lectures/PDF/lecture%2005%20computer%20architecture.pdf

So, you have 256 rows each 1 pixel thick.
You have 32 columns each 16 bits thick, or 512 columns, each 1 pixel thick.

So, to set pixel at x,y where x,y within (0-511, 0-255) :

Set (black) the y % 16 bit of the word addressed by Screen[ x*32 + y/16]

where Screen[X] implies memory address 16384 + X.

Nand2Tetris Hack Language Min Function

Make a more efficient mult:)

// finds min of R0 and R1 and stores the result in R2.
// (R0, R1, R2 refer to RAM[0], RAM[1], and RAM[2], respectively.)

// how : start off assuming R0 is smaller. Then, subtract R1 from R0 and, if 
// result is > 0 => R1 is smaller, put R1 in R2.

@R0
D=M
@R2
M=D // until we find that R1 is smaller
@R1
D=M
@R0
M=M-D // we have R0 - R1
@R0 // if > 0, then we need to put R1 in R2
D=M
@R1SMALLER
D, JGT // if R1 is smaller than R0, this will be > 0 ..
@END
0, JMP // else, we're done

(R1SMALLER)
@R1
D=M
@R2
M=D

(END)
@END
0, JMP

nand2tetris Hilf Help Shimon!

I load a program into the CPU emulator and nothing happens.

Might need to drag the lower right corners south-east a little to see the status line - you'll probably see a message there. Why the hell they don't have a View > Parser Log through the Menu just beats me. But hey, it's free :)

I do

@LOOP
JMP

and it doesn't like it. Ya, coz you HAVE to supply the core part of a C (compute) instruction always - only the jump and destination are optional. So, in this case, just do

0, JMP

Better nand2tetris. Please Shimon. Please Yaron.

In script ... illegal terminator '}' -- when your code doesn't even contain that character on that line. This happens when you run the HDLsimulator from the command line and give it an HDL file rather than a test script.

Why not provide an intelligent error message - ATLO perl saying ". Missing semicolon on previous line?" You get the idea..

Also, support for underscores would be nice. Duh :)