In stack mode, memory-reference instructions have the following form:
and operate instructions have the same format as in normal mode.
The instructions beginning with a 0 bit consist of one halfword in which three operations are specified. These operations perform arithmetic on a stack, and are as follows:
00000 NOP no operation
00001 00nnn MODE switch to the stack for values of type nnn:
000 byte 100 medium
001 halfword 101 floating
010 integer 110 double
011 long 111 quad
00010 DUP append an extra copy of the current top item
on the stack to the top of the stack
00011 DROP discard the top item on the stack
00100 ADD replace the top two items on the stack with
one item containing their sum
00101 SUB
00110 MUL
00111 DIV
01000 AND
01001 OR
01010 XOR
01011 SWAP exchange the top two items on the stack
10nnn PUSH append the value in register nnn to the top of
the stack
11nnn POP remove the top item on the stack, and place it
in register nnn
The stacks are in memory, and the scratchpad registers are used as the pointers to the stacks of the eight types.
The opcodes of the memory-reference and register-to-register instructions are split into five and two bits in the same fashion as in vector register mode, the first two bits of the opcode being moved to a later position.
The Hewlett-Packard 3000 series computers had a similar mechanism for placing two stack instructions, each six bits in length, in a single sixteen-bit word.
In stack mode, memory-reference instructions have the following form:
and the special instructions have the following format, which was also used in short page mode, for the character, packed decimal, and related instructions:
and for the other special instructions:
As these modes also use 4,096-byte pages, like the modes described on the preceding page, selective versions of these modes, which use the indirect bit to permit the use of one 32,768-byte page instead, are also available.
As with mutable scratchpad mode and stateful scratchpad mode, the instruction formats of register short page stack mode and short page stack mode are identical, and the same applies to register stack mode and stack mode.
In register stack mode, the stacks are of fixed size. The supplementary registers are used to provide the required stacks; the supplementary arithmetic/index registers provide four stacks with 16 elements each for the four fixed-point types, and the supplementary floating-point registers provide four stacks with 16 elements each for the four floating-point types.
Base register zero contains the eight four-bit stack pointers used by this mode in order.