All instructions allows to form memory address by two ways:

• as a sum of the values from two registers (second and third operands);
• as a sum of a register value (second operand) and byte immediate constant (third operand).

Such method of address calculation makes possible to realize indirect and index method of data addressing easily.

To understand the difference between signed and unsigned save please read a page about sign and unsigned data and especially about sign extension.

Code	Mnemonic	Name	Comments
A0	STB $X, $Y, $Z	store byte	the lowest byte from register X is stored to memory address [$Y + $Z] (integer overflow may occure)
A1	STB $X, $Y, Z	store byte immediate	the lowest byte from register X is stored to memory address [$Y + Z] (integer overflow may occure)
A2	STBU $X, $Y, $Z	store byte unsigned	the lowest byte from register X is stored to memory address [$Y + $Z] (no overflow!)
A3	STBU $X, $Y, Z	store byte unsigned immediate	the lowest byte from register X is stored to memory address [$Y + Z] (no overflow!)
A4	STW $X, $Y, $Z	store wyde	the lowest 2 bytes from register X is stored to memory address [$Y + $Z] (integer overflow may occure)
A5	STW $X, $Y, Z	store wyde immediate	the lowest 2 bytes from register X is stored to memory address [$Y + Z] (integer overflow may occure)
A6	STWU $X, $Y, $Z	store wyde unsign	the lowest 2 bytes from register X is stored to memory address [$Y + $Z] (no overflow!)
A7	STWU $X, $Y, Z	store wyde unsign immediate	the lowest 2 bytes from register X is stored to memory address [$Y + Z] (no overflow!)
A8	STT $X, $Y, $Z	store tetra	the lowest 4 bytes from register X is stored to memory address [$Y + $Z] (integer overflow may occure)
A9	STT $X, $Y, Z	store tetra immediate	the lowest 4 bytes from register X is stored to memory address [$Y + Z] (integer overflow may occure)
AA	STTU $X, $Y, $Z	store tetra unsigned	the lowest 4 bytes from register X is stored to memory address [$Y + $Z] (no overflow!)
AB	STTU $X, $Y, Z	store tetra unsigned immediate	the lowest 4 bytes from register X is stored to memory address [$Y + Z] (no overflow!)
AC	STO $X, $Y, $Z	store octa	register X is stored to address [$Y + $Z] (no overflow!)
AD	STO $X, $Y, Z	store octa immediate	register X is stored to address [$Y + Z] (no overflow!)
AE	STOU $X, $Y, $Z	store octa unsign	(identical to AC instruction)
AF	STOU $X, $Y, Z	store octa unsign immediate	(identical to AD instruction)

The instruction STHT and STCO will be described later.

Examples:

• instruction STBU $1, $2, $3 does the following: calculate memory address as a sum of the values from registers 2 and 3; writes the lowest byte from register 1 to memory; operation is always successful
• the only difference of instruction STBU $1, $2, 3 and the previous one is in memory address calculation: here it is a sum of the registers 2 value and constant 3; note that if the last operand is 0, instruction just write to memory address stored in register 2
• instruction STB $1, $2, $3 have some specific in comparison with the first example: an integer exception may occure if register 1 value is not in byte range between -128 and 127 excludely: for example if $1 = FFFF FFFF FFFF FFFF, operation will be successful, but for 0000 FFFF FFFF FFFF - not
• instruction STT $1, $2, $3 stores 4 lower bytes to memory address [$2 + $3] from register 1

"MMIX basics" page