The instructions to be described in this section have the formats shown in this diagram:
These instructions are 18 bits in length.
The short instructions do not include memory-reference instructions, or the subroutine jump instruction. However, while the set of 18-bit instructions is therefore not complete in itself, it is sufficient that a large proportion of the instructions in a program could be 18-bit instructions.
From two to fifteen consecutive instructions of this type may be embedded in a longer instruction, following the format below:
The lengths of these instructions, and the numbers of embedded 18-bit instructions within them, are:
Length Number of Embedded Instructions 48 2 64 3 80 4 112 5 128 6 144 7 160 8 176 9 192 10 208 11 224 12 256 13 272 14 288 15
Line 1, in the topmost diagram of the formats of the 18-bit embedded instructions themselves gives the format of the operate instructions. Integer instructions reference the integer registers, and floating-point instructions reference the floating-point registers, as might be expected.
There are 96 possible opcodes, as the first two bits of an opcode may not be both 1, as these combinations are reserved for other 24-bit instructions.
The opcodes (with the first four bits appearing along the top of the chart, and the last three bits appearing on the right) are:
0 10 20 30 40 50 60 70 100 110 120 130
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011
IB DEUH IH DEU I DEUL SW SWM SWF SWD SWQ 000 0
CB XB CH XH C X CL XL CM CF CD CQ 001 1
LB ULB LH ULH L UL LL SWL LM LF LD LQ 010 2
STB DUB STH DUH ST DU STL DUL STM STF STD STQ 011 3
AB NB AH NH A N AL NL AM AF AD AQ 100 4
SB OB SH OH S O SL OL SM SF SD SQ 101 5
MB SWB MH MEH M ME ML MEL MM MF MD MQ 110 6
DB SWH DH DEH D DE DL DEL DM DF DD DQ 111 7
The different instructions are:
For integer types:
M MULTIPLY Multiply the contents of the source and destination locations, placing the
least significant part of the result of the same length as the two input
operands in the destination location, with sign extension if that is shorter
than the length of the destination register
D DIVIDE Divide the contents of the source location by the contents of the destination
location, placing the quotient in the destination location
L LOAD Place the contents of the source operand in the destination register;
if the type involved is smaller than the register, perform sign extension
ST STORE Fill the destination location from the least significant part of the
source location
A ADD Add the contents of the source and destination locations, placing the
result in the destination location
S SUBTRACT Subtract the contents of the source location from those of the destination
location, placing the result in the source location
SW SWAP Exchange the contents of the source and destination locations
C COMPARE Subtract the contents of the source location from the contents of
the destination location, but with the operation modified so that
overflow cannot possibly result, and set the condition codes appropriately
without modifying the destination location
I INSERT Fill the least significant bits of the destination register with
the contents of the source location, leaving the rest of the destination
register unaffected
X EXCLUSIVE OR Perform a bitwise Exclusive OR operation between the contents of the source
and destination locations, placing the result in the desination location
UL UNSIGNED LOAD Fill the least significant bits of the destination register with
the contents of the source location, and clear the remaining more
significant bits of the destination register
DU DIVIDE UNSIGNED Divide the contents of the source location by the contents of the destination
location, considering both as unsigned integers, placing the quotient in the
destination location
N AND Perform a bitwise Logical AND operation between the contents of the source
and destination locations, placing the result in the desination location
O OR Perform a bitwise Logical OR operation between the contents of the source
and destination locations, placing the result in the desination location
ME MULTIPLY EXTENSIBLY Multiply the contents of the source and destination locations. Take the
full product, as an integer having twice the size as that of the source
and the destination, and:
- in the case of the halfword and integer versions of the instruction, place it in the destination
register, with sign extension in the halfword version;
- in the case of the long version of the instruction, place the most significannt half of the result
in the destination register, which must be an even-numbered register, and place the least significant
half of the result in the register following
DE DIVIDE EXTENSIBLY Divide a destination operand of twice the length of that indicated by the
instruction type (and located as the result of the MULTIPLY EXTENSIBLY
instruction) by the source operand; store the double length quotient
in the destination location (again following the MULTIPLY EXTENSIBLY
result placement) and the single length remainder in the next register
following those that are used.
Whenever a result is not wide enough to fill a register, sign extension
is performed.
Division is performed giving a result as if both operands were converted
to positive numbers before starting, with the signs then set afterwards
to give a correct result based on the actual signs of the operands. Thus
both the quotient and the remainder will be positive or zero if the dividend
and divisor have the same sign, and both will be negative or zero if they
are of opposite signs.
DEU DIVIDE EXTENSIBLY UNSIGNED This instruction also uses a dividend, and gives a quotient, at
double the length indicated by the instruction type, while the
divisor and the remainder are at the actual length the instruction
type indicates; it differs by acting on unsigned integers.
The possible integer types, and the suffixes that indicate them, are:
B BYTE An 8-bit two's complement integer
H HALFWORD A 16-bit two's complement integer
INTEGER A 32-bit two's complement integer
L LONG A 64-bit two's complement integer
The integer registers are 64 bits long, to contain the longest of these types.
The available floating-point operations are SWAP, LOAD, STORE, ADD, SUBTRACT, MULTIPLY, and DIVIDE. Their functions are basically the same as those of the corresponding integer operations, except that floating-point arithmetic is performed.
The possible floating-point types for 16-bit instructions, and the suffixes that indicate them, are:
M MEDIUM A 48-bit floating-point number (preferably aligned on 16-bit boundaries) F FLOATING A 32-bit floating-point number D DOUBLE A 64-bit floating-point number Q QUAD A 128-bit floating-point number
with their formats as indicated within this diagram:
Note that in the diagram, exponents for the types other than the 128-bit internal type are given as excess-126, excess-510, and excess-1022; documentation for the IEEE 754 standard, and most descriptions of it, refer to the exponents as excess-127, excess-511, and excess-1023 instead. This is because these accounts place the binary point of the mantissa in front of its first visible bit, while I place it in front of the hidden first bit to remain in accord with the convention used in most other floating-point formats that the mantissa is in the range [0.1). In the case of the 128-bit internal form, as the first bit of the mantissa is now the bit which would have been the hidden bit, since for the other forms, I had been placing the binary point in front of the hidden bit, the offset is consistent by remaining two less than a power of two; this would need to be the case even if I had used the normal convention for the exponents of the other formats.
The IEEE 754 standard for floating-point arithmetic includes new standard formats for 128-bit and 256-bit numbers. The format for 128-bit numbers shown above does not correspond to this standard, as it instead has no hidden bit, and is similar in its general plan to the 80-bit temporary real format used with the Intel 8087 math coprocessor. However, the exponent field in that temporary real format was only 15 bits in length; this format, instead, has an exponent field 20 bits in length./p>
This is because the format for the usual format for 128-bit numbers, while not corresponding to the new IEEE 754 standard, has been influenced by it; the exponent field is one bit longer than that of standard 256-bit floats, so that these 128-bit numbers will have the same size of exponent as an internal form for floating-point numbers suitable for supporting all the standard types of IEEE 754 floating-point numbers, including the new 128-bit and 256-bit ones.
Lines 2 through 5 of the diagram illustrate the shift and rotate short instructions. These are:
60x0xx LSLLC Logical Shift Left Long Compact
60x1xx LSRLC Logical Shift Right Long Compact
60x2xx RLLC Rotate Left Long Compact
60x3xx ASRLC Arithmetic Shift Right Long Compact
64x0xx LSLC Logical Shift Left Compact
64x1xx LSRC Logical Shift Right Compact
64x2xx RLC Rotate Left Compact
64x3xx ASRC Arithmetic Shift Right Compact
66x0xx LSLHC Logical Shift Left Halfword Compact
66x1xx LSRHC Logical Shift Right Halfword Compact
66x2xx RLHC Rotate Left Halfword Compact
66x3xx ASRHC Arithmetic Shift Right Halfword Compact
6700xx LSLBC Logical Shift Left Byte Compact
6701xx LSRBC Logical Shift Right Byte Compact
6702xx RLBC Rotate Left Byte Compact
6703xx ASRBC Arithmetic Shift Right Byte Compact
Logical right and left shifts insert zeroes; the arithmetic right shift inserts a copy of the existing value of the most significant bit into the leftmost position of the word so as to maintain the sign as either negative or non-negative.
An arithmetic left shift inserts zeroes into the leftmost end of a number regardless of its sign, just like a logical left shift, but it differs in that the overflow bit is set if a left shift results in a change of the sign of the value being shifted, instead of merely a carry out of that value; this difference is, however, not applicable to short instructions, as they may not alter the condition codes, not having space for a C bit.
In the 16-bit short instructions, there is no available separate region of opcode space for the rotate instructions, and so instead the arithmetic left shift is replaced by rotate left.
Line 6 of the diagram shows the branch instructions.
The displacement is an 8-bit signed value, in two's complement form, which may vary from -128 to +127. The displacement is units of 32 bits. A displacement of zero refers to the position immediately following the instruction.
The available branch instructions are:
7000xx NOP No Operation 7004xx BLR Branch if Low Raw 7010xx BER Branch if Equal Raw 7014xx BLER Branch if Low or Equal Raw 7020xx BHR Branch if High Raw 7024xx BNER Branch if Not Equal Raw 7030xx BHER Branch if High or Equal Raw 7034xx B Branch 7044xx BM Branch if Minus 7050xx BZ Branch if Zero 7054xx BMZ Branch if Minus or Zero 7060xx BP Branch if Plus 7064xx BNZ Branch if Not Zero 7070xx BPZ Branch if Plus or Zero 7074xx BNV Branch if No Overflow 7104xx BL Branch if Low 7110xx BE Branch if Equal 7114xx BLE Branch if Low or Equal 7120xx BH Branch if High 7124xx BNE Branch if Not Equal 7130xx BHE Branch if High or Equal 7134xx BNC Branch if No Carry 7140xx BC Branch if Carry 7144xx BV Branch if Overflow
The instructions from Branch if Minus to Branch if Plus or Zero are intended for use after floating-point operations or operations on signed integers. The instructions from Branch if Low to Branch if High or Equal are intended for use after operations on unsigned integers.
Line 7 of the diagram of short instructions shows how condition values that are invalid result instead in an additional category of instructions which affect the flags used for predicated instructions.
7210xx SFC Set Flag on Condition Set flag to 1 if condition met; leave it unaffected otherwise 7214xx CFC Clear Flag on Condition Set flag to 0 if condition met; leave it unaffected otherwise 7220xx CTF Condition to Flag Set flag to 1 if condition valid; set flag to 0 if condition not met
Line 8 of this diagram shows how an additional invalid condition value provides another special instruction:
7370xx SVC Supervisor Call
This instruction performs the equivalent of an interrupt from within software, allowing portions of the operating system not running in supervisor state to request services from the kernel, as well as possibly also allowing user programs to request services from the operating system.