8086: sketch out tricks.S for this platform

diff --git a/Kernel/platform-ibmpc/tricks.S b/Kernel/platform-ibmpc/tricks.S
index 55be713..84fbbee 100644 (file)
--- a/Kernel/platform-ibmpc/tricks.S
+++ b/Kernel/platform-ibmpc/tricks.S
@@ -1,5 +1,9 @@
 #include "../kernel-8086.def"
 
+/*
+ *     How to deal with EMM remains an interesting discussion point
+ */
+
        .arch i8086,jumps
        .code16
        .att_syntax prefix
@@ -10,43 +14,169 @@
        .globl switchout
        .globl dofork
 
+
+       /* udata transfer functions */
+
+       /* Move the udata for for udata.u_ptab into the process save slot.
+          Preserve bx. We keep udata's in a buffer but that buffer can
+          live outside the kernel so udata_ds may not be in kernel_ds
+
+          All our callers don't mind us trashing es/di/cx
+
+          NOTE: Breaks if we exceeed 128 processes */
+stash_udata:
+       /* TODO */
+       movw P_TAB__P_PAGE(%bx),%al
+       addb %al,%al
+       xchg %al,%ah            /* will be 00xx, want xx00 */
+       pushw %es
+       movw %ax,di
+       movw udata_ds,%ax
+       movw %ax,%es
+       movw udata,%si
+       movw $256,%cx
+       cld
+       rep movsw
+       popw %es
+       ret
+
+       /* Restore the udata for process %bx, preserve %bx */
+restore_udata:
+       movb P_TAB__P_PAGE(%bx),%al
+       addb %al,%al
+       xchg %al,%ah
+       pushw %ds
+       movw %ax,%si
+       movw udata_ds,%ax
+       movw udata,%di
+       movw %ax,%ds
+       movw $256,%cx
+       cld
+       rep movsw
+       popw %ds
+       ret
+
+       /* udata.u_ptab is the parent, %bx the child, preserve %bx */
+fork_copy:
+       /* Copy the data from the current instance into the new one
+          BEWARE: it is possible in the EMM case that the two processes
+          data will be in EMM in the same segment. In fact it's likely to
+          be the case. We need an optimised copier and EMM help for this
+          to do split maps (32K/32K) and blast two lots of 32K */
+       ret
+
+       /* Called to give up processor time. We will pop out of this in
+          potentially some other process, but we might also come back
+          directly */
 switchout:
        cli
        call chksigs
-       pushw %ax               /* Figure out what we actually need to save */
-       pushw %bx
-       pushw %cx
-       pushw %dx
-       pushw %es
+       movb nready,%al
+       cmpb $1,%al
+       bne slow_path
+idling:
+       sti
+       call platform_idle
+       cli
+       movb nready,%al
+       cmpb $0,%al
+       je idling
+
+       /* Ok there is something to run - but is it us ? */
+       cmpb $1,%al
+       jne slow_path
+
+       /* Fast path out */
+       ret
+
+slow_path:
+       xorw %ax,%ax
+       pushw %ax
+       pushw %ei
        pushw %di
        pushw %bp
        movw %sp,udata+U_DATA__U_SP
+
+       call stash_udata        
+
        call getproc
        push %ax
        call switchin
        jmp trap_monitor
 
 switchin:
-       push %bp
+       /* We can trash registers here because we'll restore them
+          for the process we return as */
        mov %sp,%bp
        cli
-       movw 4(%bp),%si
-       cmpb $0,P_TAB__P_PAGE_OFFSET(%si)
-       /*
+       movw 2(%bp),%bx
+       /* No MMU pointer means we are swapped out */
+       movw P_TAB__P_PAGE_OFFSET+2(%bx),%si
+       /* si is now our MMU pointer or 0 */
+       cmpw $0,%si
        jne not_swapped
-        TODO */
+       /* TODO */
+       movw P_TAB__P_PAGE_OFFSET+2(%bx),%si
+not_swapped:
+       cmpw udata+U_DATA__U_PTAB,%bx
+       je skip_copyback        /* this can happen even with idle optimisation
+                                  consider two processes ready and us winning */
+       call restore_udata
+       cmpw udata+U_DATA__U_PTAB,%bx
+       jne switchfail
+skip_copyback:
+       movb $P_RUNNING,P_TAB__P_STATUS_OFFSET(%bx)
+       xorw %ax,%ax
+       movw %ax,runticks
+       movw udata+U_DATA__U_SP,%sp
+       popw %bp
+       popw %di
+       popw %ei        
+       popw %ax
        ret
 switchfail:
-/*     call  outaxhex
+       call  outaxhex
        mov $badswitchmsg,ax
        calloutstring           
-       jmp _trap_monitor */
-
+       jmp _trap_monitor
 dofork:
-       /* TODO */
-       movw $-1,%ax
+       pushw %bp
+       movw %sp,%bp
+       cli
+       movw 4(%bp),%bx
+
+       movw P_TAB__P_PID_OFFSET(%bx),%ax
+       pushw %ax
+       pushw %bp
+       pushw %di
+       pushw %ei
+       movw %sp,udata+U_DATA__U_SP
+
+       /* These two preserve bx */
+       call stash_udata
+
+       /* Stack twice as C code is entitled to mash the first one */
+       pushw %bx
+       pushw %bx
+       call fork_copy
+       popw %bx
+       popw %bx
+
+       popw %ei
+       popw %di
+       popw %bp
+       popw %ax
+
+       pushw %bx
+       call newproc
+       popw %ax
+
+       xorw %ax,%ax            /* As child */
+       movw %ax,runticks
        ret
 
        .data
 swapstack:
        .bss 256
+       .byte "_switchin: FAIL",0
+       .even