comments and white spaces

[ysh443]

Hi
first thing i wanted to deal with , when reading the asm file, is to clean all the non relevant characters
given the following code (line numbers addedfor discussion sake):

1 //start
2 (BEGIN)
3 @18
4 JMP
5
6
7 /*end
8 (END)
9 @END
10 JMP
11 */
12
13 @MAX
14 D=MA
15 (MAX)
16 M=D+1
17
18 @18
19 0;JMP
20

cleaned code looks like this:
1 (BEGIN)
2 @18
3 JMP
4 @MAX
5 D=MA
6 (MAX)
7 M=D+1
8 @18
9 0;JMP

my questions:
1) can unconditioned jump be written in both ways? line 4 or 19
2) when removing the comment lines (1, 7 to11) and blank lines , i'm damaging the opcode line positions
   notice that my code contains both jump_to_lable and jump_to_address commands
   the labels can be easly handeled. but when using command address i'm having problem.

regards

[cadet1620]

1) There must always be a comp component in every C-instruction. The presence of the jump component is indicated by the ";". Similarly, the presence of the dest component is indicated by the presence of the "=".

2a) /* */ comments are not valid syntax for Hack assembly language; you only need to deal with end-of-line comments that begin with "//". You may support /* */ comments if you wish, but they add a bit of work to the Parser.

An easy way to deal with blank lines or lines that only have a comment is to have your Parser module's commandType() return NO_COMMAND for those lines. Then your Pass1 and Pass2 main loops can simply ignore these lines.

2b) Line numbers are irrelevant; what's important is the code address. The code address starts at 0 and is incremented for any line that generates code -- A_COMMAND and C_COMMAND only. The code address for labels is what you need to use as the value in A-commands.

Here's Max.asm showing line number, code address, and generated code (address and code in hexadecimal).

1               // This file is part of the materials accompanying the book
2               // "The Elements of Computing Systems" by Nisan and Schocken,
3               // MIT Press. Book site: www.idc.ac.il/tecs
4               // File name: projects/06/max/Max.asm
5
6               // Computes M[2] = max(M[0], M[1])  where M stands for RAM
7   0000  0000     @0
8   0001  FC10     D=M              // D=first number
9   0002  0001     @1
10  0003  F4D0     D=D-M            // D=first number - second number
11  0004  000A     @OUTPUT_FIRST
12  0005  E301     D;JGT            // if D>0 (first is greater) goto output_first
13  0006  0001     @1
14  0007  FC10     D=M              // D=second number
15  0008  000C     @OUTPUT_D
16  0009  EA87     0;JMP            // goto output_d
17  000A        (OUTPUT_FIRST)
18  000A  0000     @0
19  000B  FC10     D=M              // D=first number
20  000C        (OUTPUT_D)
21  000C  0002     @2
22  000D  E308     M=D              // M[2]=D (greatest number)
23  000E        (INFINITE_LOOP)
24  000E  000E     @INFINITE_LOOP
25  000F  EA87     0;JMP            // infinite loop

--Mark