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1987-10-30
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Microsoft Systems Journal
Volume 2; Issue 1; March, 1987
Code Listings For:
Expanded Memory
pp. 21-32
Author(s): Marion Hansen, Bill Krueger, Nick Stuecklen
Title: Expanded Memeory: Writing Programs That Break the 640K Barrier
==============================================================================
==============================================================================
Figure 3: Main Program
==============================================================================
#include <dos.h>
#include <stdio.h>
#define EMM_INT 0x67 /* EMM interrupt number */
#define GET_PAGE_FRAME_BASE 0x41 /* EMM func = get page frame
base address */
#define GET_FREE_COUNT 0x42 /* EMM Func = get unallocated
pages count */
#define ALLOCATE_PAGES 0x43 /* EMM Func = allocates pages */
#define MAP_PAGES 0x44 /* EMM Func = map pages */
#define DEALLOCATE_PAGES 0x45 /* EMM Func = deallocate pages */
#define GET_INT_VECTOR 0x35 /* DOS func = get interrupt
vector */
#define DEVICE_NAME_LEN 8 /* Number of chars in device
driver name field */
#define VIDEO_RAM_SIZE 4000 /* Total bytes in video RAM
(char/attr) */
#define VIDEO_RAM_BASE 0xB0000000 /* Video RAM start address (MDA) */
union REGS input_regs, output_regs; /* Regs used for calls to EMM
and DOS */
struct SREGS segment_regs;
unsigned int emm_status; /* Status returned by EMM */
main ()
{
unsigned int i;
long target_time, current_time;
char *video_ram_ptr = {VIDEO_RAM_BASE}; /* Pointer to video RAM */
unsigned int emm_handle; /* EMM handle */
char *expanded_memory_ptr; /* Pointer to expanded
memory */
/* Ensure that the Expanded Memory Manager software is installed
on the user's system. */
detect_emm();
/* Get a page of expanded memory. */
get_expanded_memory_page (&expanded_memory_ptr, &emm_handle);
/* Copy the current video RAM contents to expanded memory. */
memcpy (expanded_memory_ptr, video_ram_ptr, VIDEO_RAM_SIZE);
/* Clear the screen to nulls. */
memset (video_ram_ptr, '\0', VIDEO_RAM_SIZE);
/* Delay for 1 second so the user can see the blanked screen. */
time (¤t_time);
target_time = current_time + 1;
while (current_time < target_time)
{
time (¤t_time);
}
/* Restore the video RAM contents from expanded memory. */
memcpy (video_ram_ptr, expanded_memory_ptr, VIDEO_RAM_SIZE);
/* Deallocate the expanded memory page */
release_expanded_memory_page (emm_handle);
exit(0);
}
==============================================================================
==============================================================================
==============================================================================
Figure 4: Detect_EMM Subroutine
==============================================================================
detect_emm ()
{
static char EMM_device_name [DEVICE_NAME_LEN] = {"EMMXXXX0"};
char *int_67_device_name_ptr;
/* Determine the address of the routine associated with INT 67 hex. */
input_regs.h.ah = GET_INT_VECTOR; /* DOS function */
input_regs.h.al = EMM_INT; /* EMM interrupt number */
intdosx (&input_regs, &output_regs, &segment_regs);
int_67_device_name_ptr =
(segment_regs.es * 65536) + 10; /* Create ptr to device name
field */
/* Compare the device name with the known EMM device name. */
if(memcmp(EMM_device_name,int_67_device_name_ptr,DEVICE_NAME_LEN) !=0)
{
printf ("\x07Abort: EMM device driver not installed\n");
exit(0);
}
}
==============================================================================
==============================================================================
==============================================================================
Figure 5: Check_Status Subprocedure
==============================================================================
check_status (emm_status)
unsigned int emm_status;
{
static char *emm_error_strings[] = {
"no error",
"EMM software malfunction",
"EMM hardware malfunction",
"RESERVED",
"Invalid EMM handle",
"Invalid EMM function code",
"All EMM handles being used",
"Save/restore page mapping context error",
"Not enough expanded memory pages",
"Not enough unallocated pages",
"Can not allocate zero pages",
"Logical page out of range",
"Physical page out of range",
"Page mapping hardware state save area full",
"Page mapping hardware state save area already has handle",
"No handle associated with the page mapping hardware state save area",
"Invalid subfunction"
};
/* IF EMM error, THEN print error message and EXIT */
if (emm_status != 0) /* IF EMM error... */
{
emm_status -= 0x7F; /* Make error code
zero-based */
printf ("\x07Abort: EMM error = "); /* Issue error prefix */
printf ("%s\n", emm_error_strings[emm_status]);
/* Issue actual error
message */
exit(0); /* And then exit to
DOS */
}
}
==============================================================================
==============================================================================
==============================================================================
Figure 6: Get_Expanded_Memory_Page Subprocedure
==============================================================================
get_expanded_memory_page (expanded_memory_ptr_ptr, emm_handle_ptr)
unsigned int *emm_handle_ptr; /* 16 bit handle returned by EMM */
char *(*expanded_memory_ptr_ptr); /* Pointer to expanded memory
page */
{
unsigned int page_frame_base; /* Expanded memory page frame
base */
unsigned int physical_page = {0}; /* Physical page number */
/* Get unallocated pages count. */
input_regs.h.ah = GET_FREE_COUNT; /* EMM function */
int86x (EMM_INT, &input_regs, &output_regs, &segment_regs);
emm_status = output_regs.h.ah;
check_status(emm_status); /* Check for errors */
if (output_regs.x.bx < 1) /* Check unallocated page
count */
{
printf ("\x07Abort: insufficient unallocated expanded memory pages\n");
exit(0);
}
/* Allocate the specified number of pages. */
input_regs.h.ah = ALLOCATE_PAGES; /* EMM function */
input_regs.x.bx = 1; /* Number of pages to
allocate */
int86x (EMM_INT, &input_regs, &output_regs, &segment_regs);
emm_status = output_regs.h.ah;
check_status(emm_status); /* Check for errors */
*emm_handle_ptr = output_regs.x.dx; /* Get EMM handle */
/* Map the logical page into physical page 0. */
input_regs.h.ah = MAP_PAGES; /* EMM function */
input_regs.h.al = 0; /* Logical page number */
input_regs.x.bx = physical_page; /* Physical page number */
input_regs.x.dx = *emm_handle_ptr; /* EMM handle */
int86x (EMM_INT, &input_regs, &output_regs, &segment_regs);
emm_status = output_regs.h.ah;
check_status(emm_status); /* Check for errors */
/* Determine the page frame address. */
input_regs.h.ah = GET_PAGE_FRAME_BASE; /* EMM function */
int86x (EMM_INT, &input_regs, &output_regs, &segment_regs);
emm_status = output_regs.h.ah;
check_status(emm_status); /* Check for errors */
*expanded_memory_ptr_ptr =
(output_regs.x.bx * 65536)
+ (physical_page * 16 * 1024); /* Set the expanded memory
ptr */
}
==============================================================================
==============================================================================
==============================================================================
Figure 7: Release_Expanded_Memory_Page Subroutine
==============================================================================
release_expanded_memory_page (emm_handle)
unsigned int emm_handle; /* Handle identifying which page
set to deallocate */
{
/* Release the expanded memory pages by deallocating the handle
associated with those pages. */
input_regs.h.ah = DEALLOCATE_PAGES; /* EMM function */
input_regs.x.dx = emm_handle; /* EMM handle passed in
DX */
int86x (EMM_INT, &input_regs, &output_regs, &segment_regs);
emm_status = output_regs.h.ah;
check_status(emm_status); /* Check for errors */
}
==============================================================================
==============================================================================
==============================================================================
Figure 8: Kernel Module
==============================================================================
CODE SEGMENT PARA PUBLIC 'CODE'
ORG 100h
ASSUME CS:CODE, DS:DATA, ES:NOTHING, SS:STACK
max_proc_entries EQU 64
pseudo_over_struct STRUC
proc_data_segment DW ?
proc_extra_segment DW ?
proc_entry_count DW ?
proc_entry_ptr DD max_proc_entries DUP (?)
pseudo_over_struct ENDS
main PROC NEAR
MOV AX, DATA ; Segment initialization
MOV DS, AX
check_for_emm_loaded:
CALL test_for_EMM ; Use the "interrupt vector"
JE get_emm_page_frame ; technique to determine
JMP emm_err_exit ; whether EMM is loaded
get_emm_page_frame:
MOV AH, 41h ; Get the page frame base
INT 67h ; address from EMM
OR AH, AH
JZ allocate_64K
JMP emm_err_exit
allocate_64K:
MOV exp_mem_segment, BX ; Allocate 4 pages of expand-
MOV AH, 43h ; ed memory for this example.
MOV BX, 4 ; More can be allocated de-
INT 67h ; pending on the number of
OR AH, AH ; overlays to be loaded.
JZ map_64K ; Actually, in this case,
JMP emm_err_exit ; only a single page is re-
; quired because the example
; pseudo-overlay is extreme-
; ly small.
map_64K:
MOV handle, DX ; Map in the first 4 logical
MOV CX, 4 ; pages at physical pages 0
map_pages_loop: ; through 3
MOV AH, 44h ; logical page 0 at
MOV BX, CX ; physical page 0
DEC BX ; logical page 1 at
MOV AL, BL ; physical page 1
MOV DX, handle ; logical page 2 at
INT 67h ; physical page 2
OR AH, AH ; logical page 3 at
LOOPE map_pages_loop ; physical page 3
JE init_load_struct ; If additional overlays were
JMP emm_err_exit ; required, each overlay
; would be loaded after map-
; ping and a new set of
; logical pages would be
; mapped at the same
; physical pages.
init_load_struct:
MOV ES, exp_mem_segment ; Initialize pseudo-overlay
MOV DI, 0 ; environment and procedure
MOV CX, (SIZE pseudo_over_struct) ; pointer area. This struc-
MOV AL, 0 ; ture begins at the page
REP STOSB ; frame segment address.
MOV AX, (SIZE pseudo_over_struct) ; Compute the load address
ADD AX, 000Fh ; within expanded memory for
AND AX, 0FFF0h ; the overlay. The address is
MOV CX, 4 ; rounded up to the next
SHR AX, CL ; higher paragraph boundary
ADD AX, exp_mem_segment ; immediately following the
MOV parm_block.load_segment, AX ; pseudo-overlay environment
MOV parm_block.reloc_factor, AX ; & procedure pointer
; structure. This computa-
; tion takes into account
; the maximum number of
; procedure entry points
; which the pseudo-overlay
; is going to return to
; this program.
MOV WORD PTR entry_point[0], 100h ; Build .COM file entry
MOV WORD PTR entry_point[2], AX ; point
MOV AH, 4Bh ; Load the pseudo-overlay
MOV AL, 03h ; using the DOS "load
LEA DX, pseudo_over_name ; overlay" function
PUSH DS
POP ES
LEA BX, parm_block
INT 21h
JC emm_err_exit
PUSH DS ; Transfer control to the
PUSH ES ; loaded pseudo-overlays
CALL DWORD PTR entry_point ; initialization code
POP ES
POP DS
OR AH, AH
JZ call_over_procedures
JMP emm_err_exit
call_over_procedures:
MOV ES, exp_mem_segment ; As an example of passing
MOV BX, 0 ; control to a procedure
MOV DI, 0 ; existing in expanded
MOV CX, ES:[BX].proc_entry_count ; memory, each procedure con-
JCXZ deallocate_exp_memory ; tained in the overlay will
; be called in sequence.
; Obviously, a single pro-
; cedure could be called
; just as easily.
pseudo_over_call_loop:
PUSH BX
PUSH CX
PUSH DI
PUSH ES
PUSH DS
LDS AX, ES:[BX+DI].proc_entry_ptr
MOV WORD PTR CS:tp_ent_ptr[0], AX
MOV WORD PTR CS:tp_ent_ptr[2], DS
MOV AX, 123 ; Pass 2 numbers to
MOV DX, 23 ; the procedures
MOV DS, ES:[BX].proc_data_segment ; Set up pseudo-overlays
MOV ES, ES:[BX].proc_extra_segment ; segment environment
CALL DWORD PTR CS:tp_ent_ptr ; Call each procedure
POP DS
POP ES
POP DI
POP CX
POP BX
ADD DI, 4 ; Adjust index to the next
LOOP pseudo_over_call_loop ; procedure (4 bytes long)
; pointer & loop till all
; have been called
deallocate_exp_memory:
MOV AH, 45h ; Return the allocated
MOV DX, handle ; pages to the expanded
INT 67h ; memory manager
OR AH, AH
JNZ emm_err_exit
exit:
MOV AH, 4Ch ; Return a normal exit code
MOV AL, 0
INT 21h
emm_err_exit:
MOV AL, AH ; Display the fact that
MOV AH, 09h ; an EMM error occurred
LEA DX, emm_err_msg ; Go to the normal exit
INT 21h
JMP exit
tp_ent_ptr DD ? ; CS relative far pointer
; used for transfer to the
main ENDP ; procedures in the
; pseudo_overlay
==============================================================================
==============================================================================
==============================================================================
Figure 9: Procedure to Test for the Presence of EMM
==============================================================================
test_for_EMM PROC NEAR
MOV AX, 3567h ; Issue "get interrupt vector"
INT 21h
MOV DI, 000Ah ; Use the SEGMENT in ES
; returned by DOS, place
; the "device name field"
; OFFSET in DI.
LEA SI, EMM_device_name ; Place the OFFSET of the EMM
; device name string in SI,
; the SEGMENT is already in DS.
MOV CX, 8 ; Compare the name strings
CLD ; Return the status of the
REPE CMPSB ; compare in the ZERO flag
RET
test_for_EMM ENDP
CODE ENDS
==============================================================================
==============================================================================
==============================================================================
Figure 10: Pseudo-overlay Module
==============================================================================
CODE SEGMENT PARA PUBLIC 'CODE'
ASSUME CS:CODE, DS:DATA
ORG 100h
actual_proc_entries EQU 2
overlay_entry_struct STRUC
proc_data_segment DW ?
proc_extra_segment DW ?
proc_entry_count DW ?
proc_entry_ptr DD actual_proc_entries DUP (?)
overlay_entry_struct ENDS
==============================================================================
==============================================================================
==============================================================================
Figure 11: Procedure to Identify Overlay
==============================================================================
command_line_entry_point PROC NEAR
MOV AX, DATA ; Set up local data
MOV DS, AX ; segment
LEA DX, overlay_err_msg ; Display overlay error
MOV AH, 09h ; message
INT 21h
MOV AX, 4C00h ; Exit back to DOS
INT 21h
command_line_entry_point ENDP
==============================================================================
==============================================================================
==============================================================================
Figure 12: Data Segment for the Pseudo-overlay Module
==============================================================================
DATA SEGMENT PARA PUBLIC 'DATA'
sum_msg DB 0Dh, 0Ah, 'Sum of numbers = ', '$'
diff_msg DB 0Dh, 0Ah, 'Difference of numbers = ', '$'
overlay_err_msg DB 'Overlay cannot be executed via the command line$'
powers_of_ten DW 10000, 1000, 100, 10, 1
DATA ENDS
END command_line_entry_point
==============================================================================
==============================================================================
==============================================================================
Figure 13: Pseudo-overlay Data Structure Initialization Procedure
==============================================================================
initialization PROC FAR
MOV AX, DATA ; Set up a local
MOV DS, AX ; data segment
MOV AH, 41h ; Get the page
INT 67h ; frame segment
OR AH, AH ; address from EMM
JNZ error
MOV ES, BX ; Create a pointer
MOV DI, 0 ; to the expanded
; memory page frame
; segment address
MOV ES:[DI].proc_data_segment, DS ; Return local data
MOV ES:[DI].proc_extra_segment, DS ; & extra segment
; back to the kernel
MOV WORD PTR ES:[DI].proc_entry_count, 2 ; Return the number
; of local call-
; able procedures
; back to kernel
MOV WORD PTR ES:[DI].proc_entry_ptr[0], OFFSET sum ; Return a far
MOV WORD PTR ES:[DI].proc_entry_ptr[2], SEG sum ; pointer to each
MOV WORD PTR ES:[DI].proc_entry_ptr[4], OFFSET diff ; local callable
MOV WORD PTR ES:[DI].proc_entry_ptr[6], SEG diff ; procedure in the
; pseudo-overlay
; back to kernel
exit: MOV AH, 0 ; Set status in AH
; = passed
error: RET ; Return status
; in AH
initialization ENDP
==============================================================================
==============================================================================
==============================================================================
Figure 14: Procedure to Add AX and DX
==============================================================================
sum PROC FAR
ADD AX, DX ; Add numbers
PUSH AX ; Display sum message
LEA DX, sum_msg
MOV AH, 09h
INT 21h
POP AX
CALL display_result ; Display sum
RET
sum ENDP
==============================================================================
==============================================================================
==============================================================================
Figure 15: Procedure to Subtract AX and DX
==============================================================================
diff PROC FAR
SUB AX, DX ; Subtract numbers
PUSH AX ; Display difference message
LEA DX, diff_msg
MOV AH, 09h
INT 21h
POP AX
CALL display_result ; Display difference
RET
diff ENDP
==============================================================================
==============================================================================
==============================================================================
Figure 16: Procedure to Display Number in AX in Decimal
==============================================================================
display_result PROC NEAR
LEA DI, powers_of_ten
MOV CX, 5
display_loop:
XOR DX, DX ; Divide the number passed
DIV WORD PTR [DI] ; in AX by descending powers of ten
ADD AL, '0' ; Convert digit to ASCII
PUSH DX ; Output the digit
MOV DL, AL
MOV AH, 02h
INT 21h
POP AX
ADD DI, 2
LOOP display_loop
RET
display_result ENDP
==============================================================================
==============================================================================
==============================================================================
Figure 17: Data and Stack Segment for the Kernel and the Pseudo-overlay
==============================================================================
DATA SEGMENT PARA PUBLIC 'DATA'
emm_err_msg DB 'EMM error occurred$' ; EMM diagnostic message
pseudo_over_name DB 'OVERLAY.EXE', 0 ; Name of pseudo-overlay
EMM_device_name DB 'EMMXXXX0' ; Standard EMM device name
exp_mem_segment DW ? ; Temp for expanded
; memory page frame
; segment address
handle DW ? ; Temp for handle allo-
; cated to the kernel
entry_point DD ? ; Far pointer to the
; entry point for a .COM
; file
parm_block_struct STRUC ; Structure definition
load_segment DW ? ; for a "load overlay"
reloc_factor DW ? ; parameter block
parm_block_struct ENDS
parm_block parm_block_struct <> ; The actual parameter
; block
DATA ENDS
STACK SEGMENT PARA STACK 'STACK'
local_stack DW 256 DUP ('^^')
STACK ENDS
END main
==============================================================================