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# General help info for Simple Stack Machine

registers=Eight registers are available, some of which have a specific purpose. A register is private location in a processor, often faster accessible then external memory. Currently the Program Counter (PC), Stack Pointer (SP), Mark Pointer (MP) and Return Register (RR) as well as freely usable scratch registers are available, respectively identified by numbers 0..7. Registers are identified by the name R<x>, where <x> is the register number. Register with a specific purpose are also named with the name indicating their purpose.

PC=The Program Counter (PC) is used to remember what the current next instruction is. It contains the address (i.e. points to) the location of the next instruction. The machine fetches an instruction from the location pointed to by the PC. After each fetch it is automatically updated to point to the next instruction.

SP=The Stack Pointer (SP) is used to push and pop values for usage in expression evaluation. The Stack is also used to store variables. These are often accessed via the MP.

MP=The Mark Pointer (MP) is used to access local variables, allocated on the stack. Each variable is accessed using a displacement relative to the MP.

RR=The Return Register (RR) is used to return a value without placing it on a stack. Strictly seen this is not necessary but a convenience, since values also can be passed via the stack.
return register=@RR

instruction=An instruction is an encoding of some operation, executed in the machine. A set of instructions stored in memory is called the code. Some instructions have inline operands, that is, after their location in the code an extra operand is stored, a constant, e.g. in "ldc 1". In pre/post conditions this location is indicated by M[PCpre+1] since it can be found on that location. The behavior of an instruction is both informally described as well as using pre/postcondifitions.

memory=Memory stores words. A word is an 32 bits integer. Currently only a limited amount of memory words is reserver (2000), this is rather arbitrary, in the future memory size will adapt automatically to the amount needed.

stack=Stack is the part of memory used to store values needed for evaluating expressions. The stack is located after the code and grows from lower addresses to higher ones.

code=Code is the part of memory used to store instructions. It starts at address 0.

help=The Simple Stack Runner executes instructions for a hypothetical (and thus simple) machine. See memory, registers, syntax, instruction as starting points for help.

syntax=Syntax of instructions (as loaded from file) is: (label:)? (instr arg*)?. In other words, an (optional) instruction preceded by an (optional) label and followed by an argument if required. Comment may start with ";" or "//" (Java/C++ style) and ends at the end of the line. This characters are interpreted as start of comment. A label may be used as an argument. Example: "l1: beq l1 ; comment".

labels=A label is an identifier indicating a position in the code. When loading, the code location of a label is calculated (called resolution). This is done in the user interface of the program and after loading labels are not kept consistent (when adding new instructions for example).

False=Value False is encoded by a 0.

True=Value True is encoded by a -1 (all 1 bit pattern 0xFFFFFFFF). However, when testing in the context of a BRF instruction takes place, anything else than 0 is considered to be True.