CODEWITHSUNDEEP
assemblymasm
Valeriu92
Valeriu92

Reputation: 118

Assembly $ character semnification

I have a difficult exam (for me :D ) and can't find the signification of '$' character. As an example, I have the next code:

DATA    SEGMENT
vector  db    00h,10h,20h,30h,40h
        db    50h,60h,70h,80h,90h   
lv      equ   ($ - vector)/TYPE vector

Can someone tell me which is the value of lv?

Upvotes: 1

Views: 67

Answers (2)

user3079266
user3079266

Reputation:

I am pretty sure the last part:

lv equ ($ - vector)/TYPE vector

Should get you the number of elements in the vector array. The entire code is roughly equivalent to the following in C:

int vector[10] = {0x00,0x10,0x20,0x30,0x40,0x50,0x60,0x70,0x80,0x90};
int lv = sizeof(vector) / sizeof(vector[0]);

Explanation: in MASM, $ denotes the current memory offset. I.e. you've stuffed the array vector into memory, and the memory offset will be right where it ends. Thus, $ - vector will subtract the pointer to the array from the current offset, effectively giving you the array's size.

In MASM,

The TYPE operator returns the size (in bytes) of each element in an array.

(quote from http://www.c-jump.com/CIS77/ASM/Instructions/lecture.html - seems like a good MASM doc, by the way). I.e. it's equivalent to C's sizeof(vector[0]);.

Upvotes: 2

Guffa
Guffa

Reputation: 700152

The $ contains the address where the current instruction will be.

The value of lv will be the number of items in the vector data. The expression ($ - vector) calculates the number of bytes from the vector label to the place in the code where the $ is used, i.e. where lv is declared.

The length is divided by the size of the data used in vector. If you had ten words instead of bytes, then lv would still be 10, as the 20 bytes that it occupies would be divided by 2.

Upvotes: 4

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