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I am learning about Hack Assembly Language and Jump Instructions. Could anyone please give me elaborate explanations of how you could calculate the jump instructions for a given virtual machine code. I would like to understand how to find the Jump Instructions if I had the Virtual Machine Command of, for example, push constant 0.

Please give me detailed explanations so I can learn!

Example: VIRTUAL MACHINE COMMAND is 'push constant 0'.

How to get from THAT COMMAND to the following written JUMP INSTRUCTIONS:

@SP

A = M

M = 0

@SP

M=M+1

The jump instruction above is just an example, not the answer.

Ross Ridge
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Vanessa Bluebell
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1 Answers1

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Jumps are encoded in C-type instructions.

C-type instructions have the following encoding:

size (in bits):     1    2     7      3      3
                 +-----+----+------+------+------+
field            | A/C | XX | comp | dest | jump |
                 +-----+----+------+------+------+

To differentiate A-type and C-type, A/C is 0 for A-type and A/C is 1 for C-type.

If the jump field of a C-type instruction has the binary value 000, then no jump is encoded.

Otherwise, any other binary value in the jump field encodes some kind of branch.  If the condition of the branch is true, it will transfer control to the value in the A register (effectively doing pc := A), so typically, a branching instruction is preceded by an @ instruction to load the A register with the address of a label, but other approaches are possible.  If the condition of the branch is false, it will simply advance to the next sequential instruction (pc := pc + 1 instead of pc := A)

See the following presentation: https://www.coursera.org/lecture/nand2tetris2/unit-0-4-hack-language-specification-MXyAQ, at time 7:57 for a table of the C-type instruction encodings.

Erik Eidt
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