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commit Bart's patch to fix CP/M call psp:05h compatibility call along with some comments to explain the call trampoline

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git-svn-id: https://svn.code.sf.net/p/freedos/svn/kernel/trunk@1372 6ac86273-5f31-0410-b378-82cca8765d1b
This commit is contained in:
Kenneth J Davis 2009-05-01 04:25:27 +00:00
parent 497a2337ef
commit 240bcc30e2
2 changed files with 68 additions and 58 deletions
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10
hdr/process.h
View File

@ -65,11 +65,19 @@ typedef struct {
UWORD ps_exit; /* 00 CP/M-like exit point: int 20 */
UWORD ps_size; /* 02 segment of first byte beyond */
/* memory allocated to program */
BYTE ps_fill1; /* 04 single char fill */
BYTE ps_fill1; /* 04 single char fill=0 */
/* CP/M-like entry point */
/* offsets 5-9 are a far call to absolute address 0:000Ch
coded so that CP/M apps can do a near call to psp:5, does a
far call but ensures word at offset 6 is size of COM file
e.g. FEF0h by using 1MB wrap around address 0F01D:FEF0
(jmp code stored at 0:000C should be duplicated in HMA FFFF:00D0)
Note: MS-DOS has value as FEEE which wraps to 0:00BEh */
UBYTE ps_farcall; /* 05 far call opcode */
VOID(FAR ASMCFUNC * ps_reentry) (void); /* 06 re-entry point */
intvec ps_isv22, /* 0a terminate address */
ps_isv23, /* 0e break address */
ps_isv24; /* 12 critical error address */

22
kernel/entry.asm
View File

@ -65,9 +65,14 @@ segment HMA_TEXT
; VOID FAR
; cpm_entry(iregs UserRegs)
;
; This one is a strange one. The call is to psp:0005h but it returns to the
; function after the call. What we do is convert it to a normal call and
; fudge the stack to look like an int 21h call.
; For CP/M compatibility allow a program to invoke any DOS API function
; between 0 and 24h by doing a near call to psp:0005h which embeds a far call
; to absolute address 0:00C0h (int vector 30h & 31h) or FFFF:00D0 (hma).
; Note: int 31h is also used for DPMI
; Upon entry the stack has a near return offset (desired return address) and
; far return seg:offset (useless return to data at offset 0ah after far call
; in psp). We convert it to a normal call and correct the stack to appear same
; as if invoked via an int 21h call including proper return address.
;
reloc_call_cpm_entry:
; Stack is:
@ -75,10 +80,7 @@ reloc_call_cpm_entry:
; psp seg
; 000ah
;
push bp ; trash old return address
mov bp,sp
xchg bp,[2+bp]
pop bp
add sp, 2 ; remove unneeded far return offset 0ah
pushf ; start setting up int 21h stack
;
; now stack is
@ -107,12 +109,12 @@ reloc_call_cpm_entry:
; psp seg (alias .COM cs)
; return offset
;
cmp cl,024h
jbe cpm_error
cmp cl,024h ; restrict calls to functions 0-24h
ja cpm_error
mov ah,cl ; get the call # from cl to ah
jmp reloc_call_int21_handler ; do the system call
cpm_error: mov al,0
iret
iret ; cleanup stack and return to caller
;
; interrupt zero divide handler: