The MIX assembler reads MIXAL files line by line, producing, when required, a binary instruction, which is associated to a predefined memory address. To keep track of the current address, the assembler maintains an internal location counter which is incremented each time an instruction is compiled. In addition to MIX instructions, you can include in MIXAL file assembly directives (or pseudoinstructions) addressed at the assembler itself (for instance, telling it where the program starts and ends, or to reposition the location counter; see below).
MIX instructions and assembler directives6 are written in MIXAL (one per source file line) according to the following pattern:
[LABEL] MNEMONIC [OPERAND] [COMMENT]
where ‘OPERAND’ is of the form
Items between square brackets are optional, and
is an alphanumeric identifier (a symbol) which gets the current value of the location counter, and can be used in subsequent expressions,
is a literal denoting the operation code of the instruction
STA; see see MIX instruction set) or an
assembly pseudoinstruction (e.g.
is an expression evaluating to the address subfield of the instruction,
is an expression evaluating to the index subfield of the instruction, which
defaults to 0 (i.e., no use of indexing) and can only be used when
ADDRESS is present,
is an expression evaluating to the mod subfield of the instruction. Its
default value, when omitted, depends on
any number of spaces after the operand mark the beggining of a comment, i.e. any text separated by white space from the operand is ignored by the assembler (note that spaces are not allowed within the ‘OPERAND’ field).
Note that spaces are not allowed between the
MOD fields if they are present. White space is
used to separate the label, operation code and operand parts of the
We have already listed the mnemonics associated will each MIX instructions; sample MIXAL instructions representing MIX instructions are:
HERE LDA 2000 HERE represents the current location counter LDX HERE,2(1:3) this is a comment JMP 1234