User-defined labels; Indirect addressing example (p. 60,61)

[gs99]

Q1. User-defined labels are introduced on page 60:
ADD R2,R1,foo
Whereas “ADD,R2, and R1” symbols may be provided by the Machine Language, the symbol “foo” is not.
Shouldn’t there be a brief explanation of how “foo” comes to be recognized?
Perhaps we will not be using such labels.

Q2. Indirect addressing – array example (p.61)
The book says: “the jth array entry should be physically located in a memory location that is at a displacement j from the array’s base address (assuming… that each array element uses a single word). Hence the address corresponding to the expression foo[j] can be easily calculated by adding the value of j to the value of foo.”
This is confusing.

No example is provided, so I made one: foo is at 1600.
If the value of j is 0, foo[0] = 1600 + (16 * j) (16 * 0) (0) = 1600.
If the value of j is 1, foo[1] = 1600 + (16 * j) (16 * 1) (16) = 1616.
If the value of j is 2, foo[2] = 1600 + (16 * j) (16 * 2) (32) = 1632.
At this point, j + foo = (2 + 1600) = 1602, not 1632.
 
In foo[j], j is an index showing which 16-bit address (element of array) should be used.
How can the index j also equal the displacement from the base address?

[cadet1620]

gs99 wrote

No example is provided, so I made one: foo is at 1600.
If the value of j is 0, foo[0] = 1600 + (16 * j) (16 * 0) (0) = 1600.
If the value of j is 1, foo[1] = 1600 + (16 * j) (16 * 1) (16) = 1616.
If the value of j is 2, foo[2] = 1600 + (16 * j) (16 * 2) (32) = 1632.
At this point, j + foo = (2 + 1600) = 1602, not 1632.

In foo[j], j is an index showing which 16-bit address (element of array) should be used.
How can the index j also equal the displacement from the base address?

This section assumes that the size of foo's items is the same size as the memory's addressable cells, so
    foo[j] = RAM[1600+j]

The * in this section has nothing to do with multiplication.

This is a case where a picture is worth a thousand words

	Address	Content
	1599
foo -->	1600	foo[0]
	1601	foo[1]
	1602	foo[2]
	1603

--Mark

[gs99]

Thanks,

My "text" person read "each array element uses a single word" many times,
but my "calculations" person thought each element was 16 words.