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\(4.8.\)Pointer and Input, Output Control

1.Pointer

  We use A-instruction to point to a certain address. For example: 

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//   for(i = 0; i < n; i ++) {
//      arr[i] = -1
//   }

//   suppose arr = 100 and n = 10

//initialize
	@100
	D = A
	@arr
	M = D

	@10
	D = A
	@n
	M = D

	@i
	M = 0

//iteration

(LOOP)
	@i
	D = M
	@n 
	D = D - M
	@END
	D;JEQ

	//RAM[arr + i] = -1
	@i
	D = M
	@arr
	A = D + M
	M = -1
	@i
	M = M + 1
	
	@LOOP
	0;JMP

	(END)
	@END
	0;JMP

  The key step in the program is A = D + M, M = -1:

  • The A = D + M point the address of arr to the base address of arr(the initial address, 100) plus i, which match the expression arr + i.

  • By the time we say M equals -1, the register that will be affected is the register in A address.

  • Variables that store memory addresses like arr + i are called pointers.

  • Typical pointer semantics:"set the address register to the contents of some memory register"

2.Screen input

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//for (i = 0; i < n; i ++)
//    draw 16 black pixels at the 
//    beginning of row i

//    addr = SCREEN
//    n = RAM[0]
//    i = 0
//    LOOP:
//    if i > n goto END
//    RAM[addr] = -1 1111111111111111
//    addr = addr + 32
//    i = i + 1
//    goto LOOP

@SCREEN
D = A
@addr
M = D

@R0
D = M
@n
M = D

@i
M = 0

(LOOP)
	@i
	D = M
	@n
	D = D - M
	@END
	D; JGT

	@addr
	A = M
	M = -1

	@i
	M = M + 1
	@32
	D = A
	@addr
	M = M + D
	(LOOP)
	0; JMP
	
(END)
	@END
	0; JMP