The first is equate, similar to C’s:
#define len 2
in that it doesn’t actually allocate any space in the final code, it simply sets the len symbol to be equal to 2. Then, when you use len later on in your source code, it’s the same as if you’re using the constant 2.
The second is define byte, similar to C’s:
int len = 2;
It does actually allocate space, one byte in memory, stores a 2 there, and sets len to be the address of that byte.
Here’s some pseudo-assembler code that shows the distinction:
line addr code label instruction
---- ---- -------- ----- -----------
1 0000 org 1234h
2 1234 elen equ 2
3 1234 02 dlen db 2
4 1235 44 02 00 mov ax, elen
5 1238 44 34 12 mov ax, dlen
Line 1 simply sets the assembly address to be 1234h, to make it easier to explain what’s happening.
In line 2, no code is generated, the assembler simply loads elen into the symbol table with the value 2. Since no code has been generated, the address does not change.
Then, when you use it on line 4, it loads that value into the register.
Line 3 shows that db is different, it actually allocates some space (one byte) and stores the value in that space. It then loads dlen into the symbol table but gives it the value of that address 1234h rather than the constant value 2.
When you later use dlen on line 5, you get the address, which you would have to dereference to get the actual value 2.