bcopy32: refactor the bcopy routine to be able to reuse the pm part

Refactor the bcopy routine to be able to reuse the pm entry and exit
portion to call other "simple" protected-mode routines ("simple" in
the sense that no interrupt thunking support is present or expected.)

Note: consider whether or not it would be preferrable to move the
thunking support into low memory and have it be always present.

diff --git a/bcopy32.inc b/bcopy32.inc
index d0f8a46..056f29b 100644 (file)
--- a/bcopy32.inc
+++ b/bcopy32.inc
@@ -71,6 +71,13 @@ bcopy_gdt:
        desc TSS
                dw 104-1, DummyTSS      ; 30h 32-bit task state segment
                dd 00008900h            ; present, dpl 0, 104 bytes @DummyTSS
+
+               ; 16-bit stack segment, which may have a different
+               ; base from DS16 (e.g. if we're booted from PXELINUX)
+       desc SS16
+               dd 0000ffffh            ; 38h Data segment, use16, read/write,
+               dd 00009300h            ; present, dpl 0, cover 64K
+
 bcopy_gdt_size:        equ $-bcopy_gdt
 
 ;
@@ -86,13 +93,33 @@ bcopy_gdt_size:     equ $-bcopy_gdt
 ; Outputs:
 ;      ESI     - first byte after source (garbage if ESI == -1 on entry)
 ;      EDI     - first byte after target
-;      ECX     - zero
 ;
-bcopy:         push eax
-               push ebx
-               push esi
-               push edi
-               push ecx
+bcopy:         pushad
+               mov bx,pm_bcopy
+               call simple_pm_call
+               popad
+               add edi,ecx
+               add esi,ecx
+               ret
+
+;
+; This routine is used to invoke a simple routine in 16-bit protected
+; mode (with 32-bit DS and ES, and working 16-bit stack.)
+; Note that all segment registers including CS, except possibly SS,
+; are zero-based in the protected-mode routine.
+;
+; No interrupt thunking services are provided; interrupts are disabled
+; for the duration of the routine.  Don't run for too long at a time.
+;
+; Inputs:
+;      BX      - routine to execute
+;      ECX, EDX, EBP, ESI and EDI passed to the called routine
+;
+; Outputs:
+;      EAX, EBX destroyed
+;      All other registers as returned from called function
+;
+simple_pm_call:
                pushf                   ; Saves, among others, the IF flag
                push ds
                push es
@@ -102,36 +129,73 @@ bcopy:            push eax
                cli
                call enable_a20
 
-               mov byte [bcopy_gdt.TSS+5],89h  ; Mark TSS unbusy
+               mov byte [cs:bcopy_gdt.TSS+5],89h       ; Mark TSS unbusy
 
-               mov bx,ss               ; Save the stack segment value!
+               ; Convert the stack segment to a base
+               xor eax,eax
+               mov ax,ss
+               shl eax,4
+               or eax,93000000h
+               mov [cs:bcopy_gdt.SS16+2],eax
 
+               push ss                 ; Save real-mode SS selector
+       
                o32 lgdt [cs:bcopy_gdt]
                mov eax,cr0
                or al,1
                mov cr0,eax             ; Enter protected mode
                jmp PM_CS16:.in_pm
+.in_pm:
+               mov ax,PM_SS16          ; Make stack usable
+               mov ss,ax
 
-.in_pm:                mov ax,PM_DS16_4G       ; Data segment selector
+               mov al,PM_DS16_4G       ; Data segment selector
                mov es,ax
                mov ds,ax
 
-               ; Set ss, fs, and gs, in case we're on a virtual machine
-               ; running on Intel VT hardware -- it can't deal with a
-               ; partial transition, for no good reason.  However,
-               ; ss is NOT zero in general, so we have to preserve
-               ; the value.
+               ; Set fs, gs, tr, and ldtr in case we're on a virtual
+               ; machine running on Intel VT hardware -- it can't
+               ; deal with a partial transition, for no good reason.
 
                mov al,PM_DS16_RM       ; Real-mode-like segment
                mov fs,ax
                mov gs,ax
-               mov ss,ax
-
                mov al,PM_TSS           ; Intel VT really doesn't want
                ltr ax                  ; an invalid TR and LDTR, so give
                xor ax,ax               ; it something that it can use...
                lldt ax                 ; (sigh)
 
+               call bx                 ; Call actual routine
+
+.exit:
+               mov ax,PM_DS16_RM       ; "Real-mode-like" data segment
+               mov es,ax
+               mov ds,ax
+
+               pop bx                  ; Previous value for ss
+
+               mov eax,cr0
+               and al,~1
+               mov cr0,eax             ; Disable protected mode
+               jmp 0:.in_rm
+
+.in_rm:                ; Back in real mode
+               mov ss,bx
+               pop gs
+               pop fs
+               pop es
+               pop ds
+               call disable_a20
+
+               popf                    ; Re-enables interrupts
+               ret
+
+;
+; pm_bcopy:
+;
+;      This is the protected-mode core of the "bcopy" routine.
+;
+pm_bcopy:
                cmp esi,-1
                je .bzero
 
@@ -147,7 +211,7 @@ bcopy:              push eax
 
                mov cl,al               ; Copy any fractional dword
                a32 rep movsb
-               jmp .exit
+               ret
 
 .reverse:
                std                     ; Reverse copy
@@ -166,7 +230,7 @@ bcopy:              push eax
                a32 rep movsd
 
                cld
-               jmp .exit
+               ret
 
 .bzero:
                xor eax,eax
@@ -177,34 +241,6 @@ bcopy:             push eax
 
                mov cx,si               ; Write fractional dword
                a32 rep stosb
-               ; jmp .exit
-
-.exit:
-               mov ax,PM_DS16_RM       ; "Real-mode-like" data segment
-               mov es,ax
-               mov ds,ax
-
-               mov eax,cr0
-               and al,~1
-               mov cr0,eax             ; Disable protected mode
-               jmp 0:.in_rm
-
-.in_rm:                ; Back in real mode
-               mov ss,bx
-               pop gs
-               pop fs
-               pop es
-               pop ds
-               call disable_a20
-
-               popf                    ; Re-enables interrupts
-               pop eax
-               pop edi
-               pop esi
-               add edi,eax
-               add esi,eax
-               pop ebx
-               pop eax
                ret
 
 ;