source: buchla-68k/prolog/fsmain.s@ 4b63bf3

| ------------------------------------------------------------------------------
| fsmain.s -- startup code for the Buchla 700 standalone C runtime library
| Version 3 -- 1987-06-29 -- D.N. Lynx Crowe

|       This code clears 'bss' space, sets up some global variables,
|       and calls Croot(), which sets up the file system and calls main().

|       This routine should be entered with the address of the basepage
|       as its parameter.
| ------------------------------------------------------------------------------
                .text

                .xdef   start_

                .xref   Croot

                .xdef   panic
                .xdef   brk

                .xdef   _heap
                .xdef   _break
                .xdef   _pmesg

                .xdef   errno

p_bbase         =       0x18            | bss base
p_blen          =       0x1C            | bss length

                .page

| start_ -- Initial entry point -- Must be first object file in link statement
| ------    ------------------------------------------------------------------

| WARNING:  Hazardous assumptions

| We assume that:

|       the system has set the stack pointer for us.
|       the system passed us a pointer to a valid basepage.
|       the stack is above the heap.
|       BSS is located in RAM.

| If any of these assumptions is in error, we're in for serious trouble.

start_:         movea.l #0,a6           | Clear frame pointer
                movea.l 4(a7),a1        | Set pointer to base page
                movea.l p_bbase(a1),a0  | Setup to clear bss space

start1:         clr.w   (a0)+           | Clear a word
                cmpa.l  a0,a7           | See if we're done
                bne     start1          | Loop if not done yet

                move.l  p_bbase(a1),d0  | Calculate break address
                add.l   p_blen(a1),d0   | ...
                move.l  d0,_break       | Set initial break
                move.l  d0,_heap        | Set heap start

                move.l  #0,-(a7)        | Pass NULL to Croot  (no command line)
                jsr     Croot           | call Croot() routine
                addq.l  #4,a7           | ...

                move.l  #pmsg1,-(a7)    | panic(pmsg1);
                jsr     panic           | ...
                addq.l  #4,a7           | ...

hstop:          stop    #0x2000         | "Die, sucker!"
                bra     hstop

                .page

| panic -- hard halt for fatal errors
| -----    --------------------------
panic:          movea.l 4(a7),a0        | Save panic message address
                move.l  a0,_pmesg       | ...

                trap    #15             | Invoke ROMP  (we hope ...)

pstop:          stop    #0x2700         | HARD HALT
                bra     pstop

                .page

| brk -- set break value
| ---    ---------------
| WARNING:  This only works if the stack is above the heap.

brk:            cmpa.l  _break,a7       | compare current break with stack
                bcs     pstop           | actual stack overflow!

                movea.l 4(sp),a0        | get new break
                move.l  a0,d0           | compare with current stack,
                adda.l  #0x100,a0       | ... including 256-byte slop factor
                cmpa.l  a0,a7           | if (sp < a0+256)
                bcs     badbrk          |       bad break;

                move.l  d0,_break       | OK break: save the break
                clr.l   d0              | Set OK return
                rts                     | return

badbrk:         moveq.l #-1,d0          | Load return reg
                rts                     | Return

                .page

|************************************************************************
|               Data Area                                               |
|************************************************************************

                .data

pmsg1:          .asciz  "  returned from Croot() "

|************************************************************************
|               BSS Area                                                |
|************************************************************************

                .bss
                .even

_pmesg:         .ds.l   1       | panic() message string address
_heap:          .ds.l   1       | Heap start  (initial break)
_break:         .ds.l   1       | Current break location
errno:          .ds.w   1       | System error number

        .end