; Program Name: PRG_6CP.S ; Version: 1.002 ; Assembly Instructions: ; Assemble in PC-relative mode and save with a TOS extension. ; Execution Instructions: ; Execute from the desktop or from SPAWN.TTP. This program uses traps ; installed by CUSTOM.PRG. ; Program Function: ; Prints the content of register A7 during various stages of execution ; while the state of the processor is varied from user mode to supervisor ; mode, from supervisor mode back to user mode, from user mode to supervisor ; mode a second time and from supervisor mode back to user mode a second time. ; In this program, GEMDOS function $20 is used to toggle the state of the ; processor. Refer to pages 117 and 118 of the Internals book, under the ; $20 SUPER heading. Aside from the fundamental methodology of function ; invocation, two important items of information are discussed there. ; 1. GEMDOS $20 transfers the address of the user stack to the supervisor ; stack pointer (SSP). This means that while the processor is in the ; supervisor state, if that state was forced by invocation of GEMDOS $20, ; then the user stack is active and the supervisor stack is inactive. This ; means that the user stack must be large enough to accomodate supervisor ; state activity. Among other things, this activity includes the storage of ; the program counter and status register during exception processing. ; 2. Invocation of GEMDOS $20 can corrupt, and usually does as far as I can ; determine, registers A1 and D1. Information in that section of the Internals ; book indicates that only this GEMDOS function can alter the values in those ; registers. calculate_program_size: lea -$102(pc), a1 ; Fetch basepage start address. lea program_end, a0 ; Fetch program end address. trap #6 ; Return unused memory to op system. print_heading: lea heading, a0 bsr print_string ; NOTE: During this section of the program, no user stack has yet been ; assigned, therefore, the user stack address is that of the default ; user stack assigned by the system when execution commences. print_address_of_default_user_stack: lea header_1, a0 bsr print_string move.l a7, d1 ; Convert address to ASCII hexadecimal. trap #5 bsr print_string bsr print_newline invoke_gemdos_super_mode_function: move.l #0, -(sp) ; The zero turns on supervisor mode. move.w #$20, -(sp) ; Function = super = GEMDOS $20. trap #1 ; Supervisor stack pointer (SSP) returned in D0. addq.l #6, sp ; SSP = address of supervisor stack. movea.l d0, a5 ; Save SSP in scratch register. print_SSP_returned_by_GEMDOS_20: lea header_6, a0 bsr print_string move.l a5, d1 ; Convert to ASCII hexadecimal. trap #5 bsr print_string bsr print_newline print_current_SSP: ; Did GEMDOS $20 alter the address in the lea header_2, a0 ; supervisor stack pointer? bsr print_string move.l a7, d1 ; Convert to ASCII hexadecimal. trap #5 bsr.s print_string bsr print_newline bsr print_newline enter_user_mode: pea (a5) ; Restore supervisor stack pointer. move.w #$20, -(sp) ; Function = super = GEMDOS $20. trap #1 addq.l #6, sp ; NOTE: During this section of the program, a user stack is assigned. The ; user stack address is that of the label "stack". print_address_of_assigned_user_stack: lea header_3, a0 bsr.s print_string lea stack, a7 move.l a7, d1 ; Convert to ASCII hexadecimal. trap #5 bsr.s print_string bsr.s print_newline invoke_gemdos_function_again: move.l #0, -(sp) ; The zero turns on supervisor mode. move.w #$20, -(sp) ; Function = super = GEMDOS $20. trap #1 ; Supervisor stack pointer (SSP) returned in D0. addq.l #6, sp movea.l d0, a5 ; Save SSP in scratch register. print_SSP_returned_by_GEMDOS_20_again: lea header_7, a0 bsr print_string move.l a5, d1 trap #5 bsr print_string bsr print_newline print_current_SSP_again: lea header_4, a0 bsr.s print_string move.l a7, d1 ; Convert to ASCII hexadecimal. trap #5 bsr.s print_string bsr.s print_newline enter_user_mode_again: pea (a5) ; Restore supervisor stack pointer. move.w #$20, -(sp) ; Function = super = GEMDOS $20. trap #1 addq.l #6, sp print_address_of_assigned_user_stack_again: lea header_5, a0 bsr.s print_string move.l a7, d1 ; Convert to ASCII hexadecimal. trap #5 bsr.s print_string bsr.s print_newline terminate: trap #8 ; ; SUBROUTINES ; print_string: pea (a0) move.w #9, -(sp) trap #1 addq.l #6, sp rts print_newline: lea newline, a0 bsr.s print_string rts data newline: dc.b $D,$A,0 heading: dc.b 'PRG_6CP Execution Results => Content of A7:',$D,$A,$D,$A,0 header_1: dc.b ' NO USER STACK ASSIGNED',$D,$A dc.b ' Start of program: ',0 header_2: dc.b ' After GEMDOS $20 invocation: ',0 header_6: dc.b ' SSP returned by GEMDOS $20: ',0 header_3: dc.b ' USER STACK ASSIGNED',$D,$A dc.b ' After force to user mode: ',0 header_4: dc.b ' After GEMDOS $20 invocation: ',0 header_7: dc.b ' SSP returned by GEMDOS $20: ',0 header_5: dc.b ' After force to user mode: ',0 bss align ds.l 96 stack: ds.l 0 program_end: ds.l 0 end