Assembly Language

The assembly language used by the simulator closely follows the original assembly language grammar defined in the 8080/8085 Assembly Language Progamming Manual released in 1971. Sim8085 assembler does add some improvements where the differences are small and provide convinience to the programmer. Read the chapter 2 of the manual for detailed explantion on the assembly language usage.

Caution

The Sim8085 assembler tries to implement most of the features but lacks some advanced assembler features such as, Macros, relocatable code & many other directives. These may be implemented in future updates.

See Unsupported Features page for details.

Syntax Summary

; This is a comment
; It starts with a semicolon and ends at the end of the line

; All directives and mnemonics can be written in uppercase
; or lowercase (MOV, mov, Mov, ...).


; Instructions
; ============

MOV A, B   ; Instructions are written with mnemonics. In this case, it
           ; copies the content of register B into register A.
           ; Operands are separated by commas.


; Labels
; ======

START: MOV A, B ; Labels can be added to a source line to reference
                ; the memory location of this mov instruction.

JMP START       ; Here it jumps to the MOV instruction above.


; Directives
; ==========

ONES EQU 0FFH   ; EQU directive can be used to assign the
                ; value of the expression specified as operand
                ; to the name specified in the label field.


ONES SET 0FFH   ; The SET directive is identical to EQU except
                ; that the value of name specified can be
                ; altered by subsequent SET directives.

; You may think of SET as let in JavaScript and EQU as const.
; Also, note that labels for EQU and SET do not need to end with
; `:` symbol and they are called Names.


; Label    Opcode     Operands       Assembled Code
  HERE:    DB         0A3H           A3
  WORD1:   DB         -03H, 5 * 2    FD0A
  STR:     DB         'TIME'         54494D45


; Label    Opcode     Operands       Assembled Code
  FOUR:    DW         4H             0400
  STRNG:   DW         'A', 'AB'      41004241


ORG 1000H ; The ORG directive sets the location counter to the value
          ; specified by the operand expression.
          ; If no ORG directive is included before the first instruction
          ; or data byte in the program, assembly begins at location
          ; zero

Source Line Format

Each line in the program should be in the following format.

Label:/Name     Opcode    Operand    ;Comment

Some of the lines may only have comments. Label or the Name part of the line is also optional.

Label/Name Field

Labels are always optional. An instruction label is a symbol name whose value is the location where the instruc- tion is assembled. A label may contain from one to six alphanumeric characters, but the first character must be alphabetic or the special characters ”?’ or ’@’. The label name must be terminated with a colon. A symbol used as a label can be defined only once in your program.

A name is required for the SET, EQU directives.

Operand Fields

The operand may use the following four types of information,

• Register
• Register pair
• Immediate Data
• 16-bit Address

There are multiple ways to specify these which are described below,

Hexadecimal Data

Each hexadecimal number must begin with a numeric digit (0 through 9) and must be followed by the letter H or h.

;Label   Opcode   Operand    Comment
 HERE:   MVI      C, 0BAH    ;LOAD REG C WITH HEX BA

Decimal Data

Each decimal number may be identified by the letter D immediately after its last digit or may stand alone. Any number not specifically identified as hexadecimal, octal, or binary is assumed to be decimal.

;Label   Opcode   Operand    Comment
 ABC:    MVI      E, 15      ;LOAD E WITH 15 DECIMAL
         MVI      E, 15D

Octal Data

Each octal number must be followed by the letter O or the letter Q.

;Label   Opcode   Operand    Comment
 LABEL:  MVI      A, 72Q     ;LOAD OCTAL 72 INTO ACCUM

ASCII Constant

One or more ASCII characters enclosed in single quotes define an ASCII constant. Two successive single quotes must be used to represent one single quote within an ASCII constant.

;Label   Opcode   Operand    Comment
         MVI      E,'*'      ;LOAD E REG WITH 8-81T ASCII
                            ;REPRESENTATION OF *

 DATE:   DB       'TODAY"S DATE'

Labels Assigned Values

The SET and EQU directives can assign values to labels. In the following example, assume that VALUE has been assigned the value 9FH; the two MVI statements are equivalent:

;Label   Opcode   Operand    Comment
 VALUE   EQU      9FH
 Al:     MVI      D, 9FH
 A2:     MVI      D, VALUE

Labels of Instruction or Data

The label assigned to an instruction or a data definition has as its value the address of the first byte of the instruction or data. Instructions elsewhere in the program can refer to this address by its symbolic label name.

;Label   Opcode   Operand    Comment
 HERE:   JMP      THERE      ;JUMP TO INSTRUCTION AT THERE
 THERE:  MVI      D, 9FH

Expressions

All of the operands discussed earlier can be combined by to form an expression. Details around expressions are discussed elsewhere.

Register Operands

Symbol	Meaning
A	Accumulator register
B	Register B or register pair B and C
C	Register C
D	Register D or register pair D and E
E	Register E
H	Register H or register pair H and L
L	Register L
SP	Stack pointer register
PSW	Program status word (Contents of A and status flags)
M	Memory reference code using address in H and L