Assembly Source series of clock

This is the past DOS era of the compilation of source code, although has passed, but for the study of the assembly is still helpful, assembly language is just a basic programmer language, most people can grasp, not necessarily in-depth research.

Cgroup GROUP vector,codeseg
VECTOR SEGMENT at 0H
DB 6CH DUP (?) ; Filler
Time_lo DW? ;D OS Time
Time_hi DW? ;D OS Time
Vec_ip DW; CLOCK UPDATE VECTOR IP
Vec_cs DW; CLOCK UPDATE VECTOR CS
VECTOR ENDS
Codeseg SEGMENT PARA
Assume Cs:codeseg,ds:cgroup
ORG 100H
CLK PROC FAR
JMP SETUP; ATTACH to DOS
Intrpt LABEL DWORD
Int_ip DW 0; old UPDATE VECTOR IP
Int_cs DW 0; old UPDATE vecror CS
TICKS DW 0; TICK COUNTER
Scr_off DB 0,0; Screen OFFSET in BUFFER
Crt_port DW 0; Screen STATUS PORT
Flag db 0
Time DB 8 DUP (': ', 0BH); Time SAVE Area
Clk_int LABEL NEAR
PUSH AX; SAVE Registers
PUSH CX
PUSH DI
PUSH SI
PUSH DS
PUSH ES
PUSHF; and FLAGS
Call Cs:[intrpt];D o-old UPDATE INTERRUPT
MOV cx,0040h; Get SEGMENT of DOS TABLE
MOV ds,cx; Put in DS
MOV cx,cs:ticks; Get TICK COUNT
INC CX; INCREMENT IT
CMP cx,20; 01f4h; Has A MINUTE GONE by?
JB No_minute; NO, move on
Call UPDATE; YES, UPDATE CLOCK and
MOV cx,0; RESET TICK COUNTER
No_minute:
MOV cs:ticks,cx; SAVE UPDATED TICK COUNT
MOV cx,0b000h; Get Video SEGMENT
MOV es,cx; Put in ES
MOV Dx,cs:crt_port; Get CRT STATUS PORT ADDR
MOV Di,word PTR Cs:scr_off; Get screen BUFFER OFFSET
LEA Si,cs:time; Get DOS Time
MOV cx,16; SET up to move BYTES
CLI;D isable Other interrupts
Wait1:in AL,DX; READ CRT STATUS
TEST al,1; CHECK for VERTICAL Retrace
JNZ WAIT1; Wait for retrace low
MOV Ah,cs:[si]; Get-I-move
Wait2:in AL,DX; Get CRT STATUS
TEST al,1; CHECK for VERTICAL Retrace
JZ WAIT2; Wait for retrace
MOV Es:[di],ah; Move BYTE to Screen
INC DI INCREMENT INDEX
INC SI
LOOP WAIT1; Move NEXT BYTE
STI; ENABLE interrupts
POP ES; RESTORE Registers
POP DS
POP SI
POP DI
POP CX
POP AX
IRET; Return from INTERRUPT
CLK ENDP
UPDATE PROC NEAR
PUSH AX; SAVE Registers
PUSH BX
PUSH CX
PUSH DX
PUSH DS
MOV ax,0040h; Get address of DOS TABLE
MOV Ds,ax; Put in DS
MOV Ax,time_hi; Get high BYTE to DOS time
MOV flag,0; AM Flag
HOUR:CMP ax,0ch; CONVERT to HOURS
Jle H1
mov flag,1; set to PM
SUB ax,0ch
JMP HOUR
H1:aam; CONVERT to ASCII
ADD ax,3030h
LEA Bx,cs:time; Get Address of
MOV Cs:[bx],ah; SAVE HOURS-DIGIT
MOV Cs:[bx+2],al; SAVE HOURS SECOND DIGIT
MOV Ax,time_lo; Get DOS Time Low BYTE
MOV cx,8h; CONVERT to MINUTES
SHR AX,CL
MOV dx,3ch
MUL DL
SHR AX,CL
AAM; CONVERT to ASCII
ADD ax,3030h
MOV Cs:[bx+6],ah; SAVE MINUTES-DIGIT
MOV Cs:[bx+8],al; SAVE MINUTES SECOND DIGIT
mov byte ptr cs:[bx+12], ' a '
CMP flag,0 is it am?
JZ Goahead
mov byte ptr cs:[bx+12], ' P '
Goahead:
mov byte ptr cs:[bx+14], ' m '
POP DS; RESTORE Registers
POP DX
POP CX
POP BX
POP AX
Ret
UPDATE ENDP
Setup:mov ax,0; Get address of VECTOR TABLE
MOV Ds,ax; Put in DS
CLI;D Isable further interrupts
MOV AX,[VEC_IP]; Get address of the old UPDATE IP
MOV Cs:[int_ip],ax; SAVE IT
MOV Ax,[vec_cs]; Get address of the old UPDATE CS
MOV Cs:[int_cs],ax; SAVE IT
MOV Vec_ip,offset Clk_int; Put the CLK in VECTOR IP
MOV Vec_cs,cs; Put CS-CLK in VECTOR CS
STI; ENABLE interrupts
MOV ah,0fh; READ Video STATUS
INT 10H
SUB ah,8; Subtract 8 CHAR time from NCOLS
SHL ah,1; MULTIPLY by 2 for ATTRIBUTE
MOV Cs:scr_off,ah; SAVE screen Time LOCATION
MOV WORD PTR Cs:crt_port,03bah; SAVE MONO STATUS PORT ADDR
TEST al,4; CHECK for COLOR MONITOR
JNZ MONO; IF MONO, move on
ADD WORD PTR cs:scr_off,8000h; ADD COLOR offset to time offset
MOV WORD PTR Cs:crt_port,03dah; SAVE COLOR STATUS PORT ADDR
Mono:call Update;D o-I update & PRINT time
MOV Dx,offset SETUP; Get end address of NEW INTERRUPT
INT 27H; TERMINATE and remain resident
DB 117 DUP (0); Filler
Codeseg ENDS
End CLK