EMCH 367
Fundamentals of Microcontrollers
367pck S01.doc
PROGRAMMING THE MICROCONTROLLER ASSEMBLY LANGUAGE Assembly language is of higher level than machine language and hence easier to use. An assembly language code consists of a) Program statement lines b) Comment lines A program statement is a line that contains 4 fields in the following format: [
]]]]]]]], or , and , use upper case characters. When writing , use lower case. The correspond to the microcontroller opcodes. These mnemonics are found in the Motorola MC68HC11 programming reference guide and related literature. The are native to the Assembly language. A list of directives is given in Table 1. The directives that you will use often are shown in bold. Table 1 contains a value, an expression, an address, or a label that the opcodes or the directives need. The operand could be up to 4 bytes long, separated by commas. Some opcodes or directives do not require operands (inherent mode). can be hex, decimal, binary, or octal numbers. Table 2 gives the assembler symbols used to this purpose. Table 2 can use any of the operators listed in Table 3 Table 3 Assembler symbols for expressions are given in Table 4: Table 4 Other important conventions is a very powerful concept that can greatly simplify the programmer’s task. The consists of a string of alphanumeric characters that make up a name somehow meaningful to the programmer. The placement of the can be in one of the following positions: a) In the first column and terminates with a tab or blank character b) In any column and terminates with a colon (:) There are 3 different usages of the : 1) To assign the name inserted in the to a location in a program. The will be assigned the address of that location 2) To assign the value of an expression or constant to the name inserted in the using the EQU (equate) or SET directives. 3) To define the name of a subroutine (macro). Essentially, this is the same as 1), since an address (the subroutine starting address) is assigned to the label. When labels are assigned to certain addresses, one can tell the program to go to that address by referring to the label (case 1 above). Alternatively, one can use the contents of a certain address by referring to its label, just like when using variables (case 2 above). A comment is prefixed by semicolon (;).When the assembler detects an semicolon, it knows that the rest of the line is a comment and does not expect any executable instructions from it. A comment can be a separate line (comment line) or can be inserted in a program statement. A comment line can be also prefixed by an asterisk (∗). The comments, either in the comment field or as a separate comment line, are of great benefit to the programmer in debugging, maintaining, or upgrading a program. A comment should be brief and specific, and not just reiterate its operation. A comment that does not convey any new information needs not be inserted. When writing a comment, use lower case characters. A program written in Assembly language is called source file. Its extension is .ASM. When the source file is assembled, two files are generated:[<parameters>](RETURN) where: > EVB monitor prompt. Command mnemonic. <parameters> Expression or address. (RETURN) RETURN keyboard key NOTES: 1) The command line format is defined using special characters that have the following syntactical meanings: <>] Assembler/disassembler BF Block fill memory with data CALL [] Execute subroutine G [] Execute program HELP Display monitor commands MD [ []] Memory Display MM [] Memory Modify MOVE Move memory to new location [<destination>] RM [p,y,x,a,b,c,s] Register modify T [] Trace $1~$ff instructions Next few pages are detailed description and examples for each command.] where: is the starting address for the assembler operation. Assembler operation defaults to internal RAM if no address is given. Each source line is converted into the proper machine language code and is stored in memory overwriting previous data on a line-by-line basis at the time of entry. The syntax rules for the assembler are as follows: (a.) All numerical values are assumed to be hexadecimal. (b.) Operands must be separated by one or more space or tab characters. Addressing modes are designated as follows: (a.) Immediate addressing is designated by pre-ceding the address with a # sign. (b.) Indexed addressing is designated by a comma. The comma must be preceded a one byte relative offset and followed by an X or Y designating which index register to use (e.g., LDAA 00,X). (c.) Direct and extended addressing is specified by the length of the address operand (1 or 2 digits specifies direct, 3 or 4 digits specifies extended). Extended addressing can be forced by padding the address operand with leading zeros. (d.) Relative offsets for branch instructions are computed by the assembler. Therefore the valid operand for any branch instruction is the branch-if-true address, not the relative offset. Assembler/disassembler subcommands are as follows. / Assemble the current line and then disassemble the same address location. ^ Assemble the current line and then disassemble the previous sequential address location. (RETURN) Assemble the current line and then disassemble the next opcode address. (CTRL)-J Assemble the current line. If there isn't a new line to assemble, then disassemble the next sequential address location. Otherwise, disassemble the next opcode address. (CTRL)-A Exit the assembler mode of operation. where: ] where: is the starting address where user program subroutine execution begins. EXAMPLE] where: is the starting address where user program execution begins. EXAMPLE ] Display a block of user memory beginning at address 1 and continuing to address 2. EXAMPLE >MD C000 C00F C000 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF] Examine/Modify contents in user memory at specified address in an interactive manner EXAMPLE >MM C700 C700 44 66(RETURN) >MM C000 C000 55 80 C2 00 CE C4 ) [<dest>] where: [<dest>]] Where: is the number ($1~$FF) of instructions to execute. Monitor program execution on an instruction-by-instruction basis. Execution starts at the current program counter (PC). 
