A system-software-beck/02-assemblers.md

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+
+# Basic Assembler Functions
+
+## Assembler Algorithm and Data Structures
+- Two Data structures - OPTAB (Operation Code Table) and Symbol Table (SYMTAB)
+- OPTAB used to look up mnemonic codes and translate them to machine language
+- SYMTAB is used to store values (addresses) assigned to labels.
+- LOCCTR is the location counter. Initialized to the beginning address
+specified by START
+
+
+# Machine Dependent Assembler features (SIC/XE Assembler)
+- Instructions that refer to memory are normally assembled using either base
+relative mode or program-counter relative addressing mode.
+- BASE directive is used in conjunction with base relative addressing
+- If the displacement needed using base relative or program-counter relative is
+not possible, then format 4 can be used.
+
+# Look into Section 2.2.1 and the examples
+
+# Program Relocation
+- It is desirable to be able to run multiple programs at the same time, sharing
+time and memory. For this the program should be relocatable so that it can be
+loaded anywhere in the memory where there is space for it.
+- Assembler does not know before hand where the program might be loaded, but it
+does know the parts that have to be modified during loading.
+
+![Example of Program relocation](img/program-relocation-example.png){width=60%}
+
+- Solution
+  1. When Assembler generates object code, JSUB is placed relative to the start
+  of the program. That is why location counter is initialized to 0
+  2. Assembler will also include the necessary information for the loader,
+  instructing it to add the beginning address of the program to the address
+  field in JSUB instruction
+
+- A Modification Record is added in the object code to instruct the loader of
+the required modifications required during loading
+- Modification Record
+```
+      Col. 1   (1): M
+      Col. 2-7 (6): Starting location of the address field to be modified,
+                    relative to the beginning of the program (hexadecimal)
+      Col. 8-9 (2): Length of the address field to be modified, in half bytes
+                    (hexadecimal)
+```
+- Modification record for JSUB example Fig. 1: `M00000705`
+- The parts of the program that need modification are the ones that used direct
+  addressing and direct addressing is only possible in format 4
+- This is also the advantage of relative addressing - it doesn't require
+modification during relocation
+
+# Machine Independent Assembler features
+
+## Literals
+- Difference between literal and immediate addressing. With immediate addressing
+the operand value is the part of the actual machine instruction but with
+literals, the assembler first generates the specified value as a constant at
+some memory location and the address of this constant is used as the target
+addressed for the machine instruction.
+- All of the different literals used in the program are gathered together in a
+**literal pool** at the end of the program.
+- In some cases it is desirable to store the literals into a pool at some other
+location. This can be achieved using **LTORG**. When **LTORG** is encountered,
+the assembler creates a literal pool that contains all of the literal operands
+used since previous LTORG or the beginning of the program. We use LTORG
+directive when it is desirable to have the literals close to the instruction
+- Denoted be '=' prefix. eg `=C'EOF'` or `=X'05'`
+- Structure needed to handle literals - literal table (LITTAB)
+
+
+## Symbol-Defining Statements
+
+- Symbols defined till now (before EQU being introduced) where using labels.
+The value of a label is the address of the statement where it appears.
+- `EQU` directive gives a way to assign values to these labels/symbols
+```
+        symbol     EQU     value
+```
+- `EQU` is mainly used to establish symbolic identities so that the source
+program is readable.
+- `ORG` is used to reset the value of the location counter.
+```
+        ORG        value
+```
+- This will reset the value of the location counter to value. (see an example
+of how this is used on page 73)
+- Restrictions while using `EQU` and `ORG`
+  - All symbols used on the right hand side of the statement must
+  have been defined previously in the program. This applies both for EQU and 
+  ORG. This is a common *forward-reference* problem and cannot be resolved by
+  even a two-pass assembler as in a two-pass assembler it is required to define
+  all symbols during pass 1.
+
+
+## Expressions
+- Assemblers allow to use expressions wherever we can use a single operand.
+- Assemblers allow the use of `+, -, *` and `/`. The terms in the expression can
+be constants, user-defined symbols or special terms (like the current value of
+the location counter - `*`)