Instruction Set and Addressing Modes

A list of instructions and addressing modes for the 6811 chip is available in three publications by Motorola, "MC68HC11 E Series Programming Reference Guide", "MC68HC11 Reference Manual", and "MC68HC11 E Series Technical Data". Each of these are available at little or no charge by calling the Motorola University Relations office at (512) 505-8836. Having all three is very useful.

There are four categories of instructions: instructions that transfer date from memory to the registers or from the registers to memory; instructions that transfer data between registers; instructions that operate on data in the registers; and instructions that branch or jump. For example, to complete the task of moving a variable named VAR1 into a register, and VAR2 into another register, adding them together, and saving the result in Var1 if no overflow occurred or saving 0 in Var1 if an overflow did occur we could use the following instructions: 


	LDAA	VAR1		; load var1 into acc a
	LDAB	VAR2		; load var2 into acc b
	ABA			; add acc b to a, result in acc a
	BVS	OVERFLOW 	; Branch if overflow occurred
	STAA	VAR1		; Store result of add into var1
	BRA	DONE	
OVERFLOW: LDAA	#0		; if V then load a zero into acc a
	STAA	VAR1		; store zero into var1
DONE:	continue with code
An assembler will convert the above mnemonics (easy for humans to remember) into opcodes and data in a format which would look like just a bunch of ones and zeros to humans (but computers sure like it) Each line above will become one or two bytes of information in the assembled object file. We call a file of mnemonics which the programmer writes the source file.

Four addressing modes are exemplified above. The first two instructions both use the direct addressing mode. When a assembler assembles the above, it should know where in memory VAR1 and VAR2 reside (it should know because you should tell it earlier in the program). The assembler will substitute the memory address in place of the name so that the load instruction may directly access an area of memory. Secondly the load instruction found on the same line as the OVERFLOW label uses the immediate addressing mode where the data is not in RAM but will be assembled immediately into the program code, that is, the value of zero is placed immediately after the opcode for ldaa into the program code. Thirdly the ABA instruction exemplifies the inherent addressing mode. All information about where to find its operand is inherently contained in the opcode itself. And finally, the fourth addressing mode may be found in both branch instructions. The programmer wrote branch to a label, and this will become the relative addressing mode. The assembler will see the branch to a label type of instruction, figure out how far away the label is, and encode that distance as an offset to be placed directly into the object file. For example, the BRA DONE instruction will assemble as one byte for the opcode for BRA and one byte for the relative offset of how far it is to get to the label "DONE". During execution, when the program counter gets to BRA DONE, the one byte offset will be added into the PC so that the next instruction to be executed will be at the label "DONE:". This addressing mode is called relative because the offset was calculated as a value relative to the current position in memory, not as a Direct value to replace the PC with. Branch instructions always produce relative addresses (one byte long) to be added to the current PC, but JMP instructions will always produce direct addresses (two bytes long) to replace the current PC with.

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