XSM Execution Environment

• 20 General Purpose Registers (R0-R19)
• Stack pointer, Base pointer, Instruction Pointer
• 5120 words address space

Text Size, Data Size, Heap Size and Stack size indicates the sizes of Text, Data, Heap and Stack regions to be allocated by the OS loader when the file is loaded for execution.

The present eXpOS virtual address space model requires that the data and stack must be in the same memory area and must be managed by the compiler / application program (this means that the program must contain the code to initialize the value of the stack pointer). The value of Data Size field is ignored. Moreover, the eXpOS loader sets the size of text region to 2048 words and stack region to 1024 words in memory irrespective of the values present in the header.

If the Runtime Library must be included when the file is loaded, the Library Flag is set to 1 in the executable file. If this flag is not set then neither memory is allocated for the heap nor the library linked to the address space of the process at execution time.

MOV Ri, Rj # register addressing
MOV Ri, INTEGER/STRING # immediate addressing
MOV Ri, [Rj] # register indirect addressing
MOV [Ri], Rj
MOV [LOC], Rj # Direct addressing
MOV Rj, [LOC]

OP Ri, Rj
OP Ri, INTEGER # add, sub, mul, div, mod
OP Ri # INR, DCR

OP Ri, Rj # LT, GT, GE, LE etc.

JZ Ri, target_address
JNZ Ri, target_address
JMP target_address

PUSH Ri # SP <- SP+1, Memory[SP] <- Ri
POP Ri # Ri <- Memory[SP], SP <- SP-1

Subroutine Instructions

These instructions manage program control flow for subroutines, using the stack to save and restore the return address. Each instruction is two memory words long.

CALL target_address/Register # Memory[SP] <- IP + 2, IP <- target_address
RET # IP <- Memory[SP]
        SP <- SP-1

BKRP -> Debug mode

INT 6 -> Read INT 7 -> Write INT 10 -> Exit

• Headers must be set, so for each xexe file, 8 words are reserved

  0
  2056 
  0
  0
  0
  0
  0
  BKRP
  MOV R0, 3
  MOV R1, 2
  ADD RO, R1
  

Command

./xsm -l library.lib -e target_file.xsm --debug

System Calls

• Printing a number is a privileged operation
• The return values must be pushed into the stack

  MOV SP, 4095 # SP increments immediately before a PUSH Operation
  

For example

Order	Assembly Code	Value/Source	Purpose
5th Word	MOV R2, 5 then PUSH R2	5	System Call Number: Tells the kernel which service to run (Write is 5 in the argument convention, not the interrupt number 7).
4th Word	MOV R2, -2 then PUSH R2	-2	Argument 1: File Descriptor (conventionally -2 for console Write).
3rd Word	PUSH R0	The 5 from R0	Argument 2 (The Data): The actual data value (5) to be printed.
2nd Word	PUSH R2	R2 (Blank)	Argument 3: A blank argument/placeholder.
1st Word	PUSH R2	R2 (Blank)	Storage for Return Value: A placeholder where the kernel will write the system call's return code (0 for success).

After that interrupt call, the stack must be cleared.

System Call	System Call Number	Interrupt Routine Number	Argument 1	Argument 2	Argument 3	Return Values
Read	7	6	-1	Buffer (int/str)*	-	0 - Success
-1 - File Descriptor given is invalid
-2 - Read error
Write	5	7	-2	Data+	-	0 - Success
-1 - File Descriptor given is invalid
Exit	10	10	-	-	-	-