LT4335 stack instructions. Version of 6 July 2011. Home

Some stack instructions apply to only the 64 registers within the CPU, but others can also involve the stack overflow area. The mnemonics begin with "P" instead of "S", because "S" is already used for string operations. The "P" was chosen because it is the first letter of verbs traditionally associated with last-in-first-out data structures: push, peek, pop and pull.

Guide to the mnemonics:

• PF: stack flush
• PI: stack information
• PD: stack discard
• PS: stack swap
• PO: stack obtain
• PR: stack rotate

In many opcodes appears u⁶, a six-bit unsigned integer whose value is denoted u; it necessarily lies in 0…63. Similar is v as the value of v⁶.

Any operation that pushes may lead to a writing into the stack overflow area; any operation that pulls may invoke reading of the overflow area. The CPU may transfer stack data between registers and the overflow area at unpredictable times. These transfers are not subject to the synchronization option of the get and put instructions, because the stack overflow area is separate from genspace. Additionally, the manner of formatting data within the overflow area is unpredictable.

The next instruction writes all the CPU registers to the overflow area. In general, this needs to be done only on a context switch.

PF	stack flush	?
	Pulled: none Pushed: none

The next instructions give information about the stack.

PIT	stack information, total size	?
	Pulled: none Pushed: #0 signed integer: the quantity of items in the stack associated with the current task, CPU and overflow combined
PIR	stack information, remaining space	?
	Pulled: none Pushed: #0 signed integer: the quantity of bits, in the stack overflow area, that would still be available if the stack were flushed.

The next instructions discard items from the top of the stack within the CPU. If more then 64 items are to be removed, the instructions also discard from the stack overflow area. An exception is thrown if the stack (CPU and overflow combined) does not contain at least the requested number of items.

PDE	stack discard, operand embedded	u8_?	PDR	stack discard, operand in register	08_?
	Pulled: a variable quantity of items to be discarded Pushed: none The leftmost 8 bits of the opcode are an unsigned integer indicating how many stack items are to be discarded. If zero, PDR is performed instead.			Pulled: • #0 nonnegative signed integer: number of items to remove from the stack, not counting #0 (throws exception on negative or null) • a variable quantity of items being discarded Pushed: none

The next instructions exchange registers among the 64 within the CPU; there is no instruction to exchange registers in the overflow area. PS1 exchanges two registers. PS2 exchanges two pairs of registers; although intended for manipulating floats, it may be used for registers containing anything else, as the LT4335 does not enforce data typing. Meta-registers are swapped, also.

PS1	stack swap, single registers	u6_v6_?	PS2	stack swap, double registers	u6_v6_?
	Pulled: none Pushed: none Exchanges the contents of #u and #v.			Pulled: none Pushed: none Exchanges the contents of #u and #v, as well as #(u+1) and #(v+1). Throws an exception if u or v equals 63, or if u and v are consecutive integers.

The next instructions push a copy of one or two registers or meta-registers.

PO1	stack obtain, single register	u6_?	PO2	stack obtain, double register	u6_?
	Pulled: none Pushed: A copy of #uold #uold is not deleted, but after the push its address is #(u+1).			Pulled: none Pushed: • #0: a copy of #uold • #1: a copy of #(u+1)old An exception will be thrown if u equals 63.
POM1	stack obtain, single meta-register	u6_?	POM2	stack obtain, double meta-register	u6_?
	Pulled: none Pushed: • #0 signed integer: a copy of ##uold #0new = ##uold = ##(u+1)new.			Pulled: none Pushed: • #0: a copy of ##uold • #1: a copy of ##(u+1)old #0new = ##uold = ##(u+2)new #1new = ##(u+1)old = ##(u+3)new An exception will be thrown if u equals 63.

In stack rotation, the top (i.e. most recently pushed) u items will be rearranged. Let r be a register number in 0…(u − 1). Then for every r, #r_new = #((r+v) mod u)_old. Of course, the meta-registers will be correspondingly rearranged. No register contents will be lost or duplicated.

PR	stack rotate	u6_v6_?
	Pulled: none Pushed: none