ashos_lc3b.ah

单总线开源CPU-LC3的简单操作系统。

;	Ash LC-3b Operating System 1.00
;	(c) Ashley Wise, 2003
;	[e-mail]
;
;	This file contains the extern symbols for the OS functions.
;
;	AshOS_LC3b.asm must be linked with the user code.
;	AsmOS_LC3b.ah should be be included in the user code.
;	R0 is used for passing values to/from OS functions.
;	R6 is expected to be the User Stack Pointer (USP).
;	R7 is overwritten by the TRAP instruction (return address).
;	All other registers are preserved.
;	Remember to NULL-terminate strings.

DEFINE KBSR 4xfe00
DEFINE KBDR 4xfe02
DEFINE DSR 4xfe04
DEFINE DDR 4xfe06
DEFINE MCR 4xfffe

; Vector tables
; All of the functions below are vectors in these tables.
; They are addresses of function pointers
EXTERN TrapVectorTable
EXTERN ExceptionVectorTable
EXTERN InterruptVectorTable

; Note that in order for the trap functions to work correctly,
; the user code must have preserved R6 as the User Stack Pointer
; (USP). R6 is initialized to this value when the simulator boots.
; If you clobber R6, trap functions might not work.
; You can define NO_USP (as a global define, not in your code)
; which will tell the OS to use local storage rather than the USP.
; However, due to a limitation of the LC-3b ISA, the TRAP functions
; will clobber any value in R5 to achieve this. So if you define
; NO_USP, don't expect R5 to be preserved.
MACRO Push(Reg)
	add r6, r6, -2
	str reg, r6, 0
END
MACRO Pop(Reg)
	ldr reg, r6, 0
	add r6, r6, 2
END

; GETC function
; Reads a single character from the keyboard. The character is not echoed
; onto the console. Its ASCII code is copied into R0. The high eight bits
; of R0 are cleared.
EXTERN GETC

; OUT function
; Write the character in R0[7:0] to the console.
EXTERN OUT

; PUTS function
; Write the string pointed to by R0 to the console.
; Make sure the string is NULL-terminated!
EXTERN PUTS

; IN function
; Print a prompt to the screen and read a single character from the keyboard.
; The character is echoed onto the console along with a newline, and its
; ASCII code is copied into R0. The high eight bits of R0 are cleared.
EXTERN IN

; HALT function
; Halt execution and print a message to the console.
EXTERN HALT

; The start of the user code space
EXTERN _MAIN

; Privilege exception function
EXTERN Privilege

; Illegal Instruction exception function
EXTERN IllegalInstr

; Keyboard interrupt function
EXTERN Keyboard