Page 33 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 2: Introduction to Assembly Language

where we have to refer to a location, either because it is the target of a branch instruction or Notes
because it contains data that must be loaded or stored. Consequently, if we have not made any
programming mistakes, and if we identify all the labels, we will have identified all the symbolic
addresses used in the program.
The above paragraph assumes that our entire program exists between our .ORIG and .END pseudo-
ops:
The assembler examines each instruction in sequence, and increments the LC once for each
assembly language instruction. If the instruction examined contains a label, a symbol table entry
is made for that label, specifying the current contents of LC as its address. The first pass terminates
when the .END instruction is encountered. The first instruction that has a label is at line 13. Since
it is the sixth instruction in the program and the LC at that point contains x300A, a symbol table
entry is constructed thus:

Symbol Address
TEST x300A

The second instruction that has a label is at line 20. At this point, the LC has been incremented to
x3018. Thus a symbol table entry is constructed, as follows:

Symbol Address
GETCHAR x3018

At the conclusion of the first pass, the symbol table has the following entries:
Symbol Address
TEST x300A
GETCHAR x3018
OUTPUT x301E

ASCII x3028
PTR x302A
The Second Pass: Generating the Machine Language Program
The second pass consists of going through the assembly language program a second time, line by
line, this time with the help of the symbol table. At each line, the assembly language instruction
is translated into an LC-3b machine language instruction.
Starting again at the top, the assembler again discards lines 01 through 09 because they contain
only comments. Line 0A is the .ORIG pseudo-op, which the assembler uses to initialize LC to
x3000. The assembler moves on to line 0B, and produces the machine language instruction
0101010010100000. Then the assembler moves on to line 0C.
This time, when the assembler gets to line 0C, it can completely assemble the instruction since it
knows that PTR corresponds to x302A. The instruction is LEA, which has an opcode encoding of
1110. The Destination register (DR) is R3, that is, 011.
PCoffset is computed as follows: We know that PTR is the label for address x302A, and that the
incremented PC is LC+2, in this case x3004. Since PTR (x302A) must be the sum of the incremented
PC (x3004) and twice the sign-extended PCoffset (since the offset is in words and memory is
byte-addressable), PCoffset must be x0013 by putting Address Binary.
In order to use the LEA instruction, it is necessary that the source of the load, in
this case the address whose label is PTR, is not more than +512 or –510 memory
locations from the LEA instruction itself.

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