CVASM

CVASM (ClearViewAssembler)

TechTools PICmicro® MCU Assembler

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General Information

Generating Assembly Listings

Command-Line Options

Assembler Basics

Addressing Definitions

Default Symbol Tables

Instruction Set

Directives

General Information

If you already know how to use an assembler, you’ll probably find our’s to be quite simple. However, if you have little or no experience with assemblers, you should read the following before attempting to use the assembler.

When you write programs for PIC microcontrollers, you’ll use a text editor to create source code. Source code is the format that you’re accustomed to looking at; it contains English-like labels, instructions, and data.

Before your source code can be used by a programmer, emulator, or other development tool, it must be converted into hex code. Hex code is the "machine-readable" version of source code; it contains instructions and data in the form of hexadecimal data, which can be executed by the PIC.

An assembler is a piece of software that converts source code into hex code.

For instance, this line of source code:
CALL SENDBYTE ;Call send routine

assembles into just two bytes of hex code:
2420h

It’s possible to write programs directly in hex code, using the individual machine codes that make up each instruction. However, most people find it preferable to use an assembler.

The TechTools assembler (CVASM16 ) is unique in its ability to accept two instruction sets: our own 8051-like instruction set, and the original Microchip instruction set. Many customers appreciate the TechTools instruction set, because it resembles other processors. However, there are certainly customers who prefer the Microchip instructions.

If you plan to use source code in the Microchip format, please note that while our assembler will accept their basic instructions, it will not accept various other aspects of their source code. The syntax of numbers is sometimes different, and our assembler does not recognize Microchip assembler directives and macros. If you plan to write a lot of code in the Microchip format, then you may wish to use the Microchip assembler (MPASM).

Generating Assembly Listings

It’s possible to have the assembler create an "assembly listing" of your program. An assembly listing is a duplicate of the source code, but with hex code information (line number, address, opcode, & data) preceding each original line. To have the assembler create a listing file, simply add "/L" after the filename:

CVASM16 filename /L

Assembles text file filename.src into hex file filename.obj and creates a listing file called filename.lst.

Command-Line Options

The PIC assembler has several options which can be invoked when it is run. These command-line options are shown below:

CVASM16 filename
Assembles text file filename.src into hex file filename.obj.

CVASM16 filename.xxx
Assembles text file filename.xxx into hex file filename.obj.

CVASM16 filename /L
Assembles text file filename.src into hex file filename.obj and creates a listing file called filename.lst.

CVASM16 filename /M
Creates a generic Microchip compatible HEX file (.HEX) instead of the TechTools format HEX file (.OBJ).

CVASM16 filename /S
Assembles text file filename.src into hex file filename.obj, but suppresses TechTools device data normally included in the file. (This may be useful with other tools that won’t accept any extra data.)

Assembler Basics

The purpose of the assembler is to convert assembly language source code into hex code. The assembler accomplishes its task in two passes:

Pass 0 - The source code is scanned in an attempt to resolve all symbols. This is possible if all origin, define space, and equate directives can be resolved (equated symbols may be referenced by origin or define space). All other symbols can be resolved by byte-offsets which are determined by the mnemonic/operand combinations. If Pass 0 is successful, the assembler will advance to Pass 1. If Pass 0 is unsuccessful, a list of errors will be shown and assembly will be aborted.

Pass 1 - The source code is scanned once more in order to assemble the hex code. Since all symbols were resolved in Pass 0, all instructions and miscellaneous directives can be fully resolved in Pass 1. If Pass 1 is successful, a hex file containing the assembled code will be created. If Pass 1 is not successful, a list of errors will be shown and assembly will be aborted.

In addition to a hex file, the assembler can be used to generate an assembly listing. An assembly listing shows line numbers, equated values, addresses, data, and original source code (for more information, see the section "Generating Assembly Listings" in TechTools PIC Tools Manual).

Addressing Definitions

Throughout your programs, you’ll refer to bits and bytes by their addresses. Depending on the instruction being used and the item being referred to, the address will be given in one of the following forms:

addr8 An 8-bit address (on 16C5x devices, the lower half of the current 512-word page).

addr9 A 9-bit address (on 16C5x devices, within the current 512-word page).

addr11 An 11-bit address (anywhere in program memory in a device with 2K of memory).

addr12 A 12-bit address (anywhere in program memory in a device with 4K of memory).

bit An address for bitwise operations
(Example: PortC.3 = bit 3 of port C)

fr A file register (RAM) address.

rel A relative address ranging from -7Fh to +80h.

literal An immediate 8-bit value.

Data Types

Eight data types are allowed in the assembler. These data types are:

Assembly address (origin)
EEPROM address (origin)
Local symbol/label
Symbol/label
ASCII value
Binary value
Decimal value
Hex value

The examples below show various data types:

18h (Hex value 18)

100 (Decimal value 100)

0A7h (Hex value A7)

1011b (Binary value 1011)

‘A’ (ASCII value for the letter A- 65 decimal)

Start (Label called Start)

:loop (Local label called :loop)

$ (Current assembly address)

% (Current EEPROM address)

In addition to single-character text, entire strings can be generated using the RETW instruction:

retw ‘The fox jumped over the lazy dog’

Expressions

Mathematical expressions are used in many instructions. These expressions may be created using the following operators:

& (Logical AND)
/ (Divide)
|   (Logical OR)
<< (Shift left)
^ (Exclusive OR )
>> (Shift right)
+ (Add )
< (High byte)
-    (Subtract )
> (Low byte)
*   (Multiply )
. (Bit address)

Some example expressions:

setb PortA.0 Set bit 0 on port A.

mov Count+3,#88h ;Store 88h in location Count+3.

ds N*2 ;Define empty space of Nx2 bytes.

All expressions are resolved strictly from left to right. Please make note of this, since it may affect the result of expressions. For instance, the expression 5+2*4 would normally be resolved as (2*4)+5, for a result of 13. Since our assemblers resolve expressions strictly from left to right, however, the example would be resolved as (5+2)*4, for a result of 28.

Symbols & Labels

Symbols are used to name locations and values within your program. Many people refer to address symbols as "labels", but both have the same effect. For instance, by assigning a symbol to the start of an important routine, you can later call that routine by its name, rather than its address. And by giving a symbol to a common value, you can refer to it by its name. Rather than typing "212" many times in your program, you can type HOT = 212 at the beginning of your program, and then use HOT wherever you need it. This is also quite handy if your idea of what’s "hot" changes. By changing the symbol definition, you can easily redefine HOT to be a different temperature.

Symbols may be up to 32 character long. They must begin with a letter or underscore (_) and must contain only letters, numbers, underscores, and colons. Further, if you’re labeling an address (such as the start of a routine), the label must start at the beginning of the line.

Here are some examples of valid symbols:

min_count = 20h

maximum_count = 21h

begin mov min_count,#05h

Local Labels

By default, labels are global, which means that they can be "seen" from anywhere in your program. Sometimes, however, you may want to use a local label, which can only be "seen" within a limited part of your program (the area in which the local label can be seen starts at the preceding global label, and continues up to the following global label).

Local labels have the same syntax rules as global labels, except they must begin with a colon (:), and must be referenced with a colon. Local labels can be referenced from from outside their normal area by referring to preceding global label name:local label name.

The following code demonstrates how to use the local label :loop for common looping purposes within two globally-labeled routines.

(global label Routine1)
Routine1 mov count,#100

(local label :loop)
:loop call send_a

(jump to line 2)
djnz count,:loop

ret

(global label Routine2)
Routine2 mov count,#200

(local label :loop)
:loop call send_b

(jump to line 6)
djnz count,:loop

ret

(global label Routine3)
Routine3 mov count,#250

(jump to line 6)
jmp Routine2:loop

Default Symbol Tables

When the assembler is started, its symbol table is initialized with various PIC symbols from the device "Include" file, such as C for the Carry register, RA for Port A, etc. This saves you from having to define every register and bit in the PIC.

If you’d like to see the symbol table for a particular PIC, open the coresponding "include" file in TDE or a text editor.