ASM: Revised Chekerboard program, now with flashing squares

I revised by checkerboard ASM program to have the white colored squares fade in and out. I wanted to do this for a few reasons. It shows a palette shift, which I have demonstrated previously, but it also shows how to do a time delay in ASM. The time delay turned out to be a little trickier than I thought. I tried using the DOS Get Time interrupt, but if you don’t test all the registers it returns and try to get away with just the DX register, you end up with unacceptably long delays when minutes change or hours change. I ended up using the system BIOS call to get the system clock ticks. The same problem will arise after midnight, but that can easily be overcome by testing the appropriate flag which gets set when midnight occurs. Again, I have included comments, and I also included the addresses that DOS Debug displays. If I do anymore ASM programs, I thing I will use an assembler going forward. Its easier than having to specify jumps by address, which change when I add something new to a program.

--initialize video mode to 320x200x256
13F2:0100 B81300        MOV     AX,0013
13F2:0103 CD10          INT     10
point data segment and extra segment to video memory
13F2:0105 B800A0        MOV     AX,A000
13F2:0108 8ED8          MOV     DS,AX
13F2:010A 8EC0          MOV     ES,AX
13F2:010C 31FF          XOR     DI,DI
--initialize registers with values of palette to use
--use word instead of bytes for the stosw instruction
13F2:010E B80F0F        MOV     AX,0F0F
13F2:0111 BB0000        MOV     BX,0000
--initialize to the number of veritical squares
13F2:0114 B90800        MOV     CX,0008
13F2:0117 51            PUSH    CX
--start execution, logic is explained in previous post
13F2:0118 B9C800        MOV     CX,00C8
13F2:011B 51            PUSH    CX
13F2:011C B91400        MOV     CX,0014
13F2:011F F3            REPZ
13F2:0120 AB            STOSW
13F2:0121 50            PUSH    AX
13F2:0122 89D8          MOV     AX,BX
13F2:0124 5B            POP     BX
13F2:0125 59            POP     CX
13F2:0126 E2F3          LOOP    011B
13F2:0128 50            PUSH    AX
13F2:0129 89D8          MOV     AX,BX
13F2:012B 5B            POP     BX
13F2:012C 59            POP     CX
13F2:012D E2E8          LOOP    0117
--get ready to ramp down the white palette entry
--do this 3fh times
13F2:012F B93F00        MOV     CX,003F
13F2:0132 B3FF          MOV     BL,FF
13F2:0134 FECB          DEC     BL
--initialize with the port to tell video card we are changing
--a pallete entry. This should be changed at some point to
--actually retrieve the pallete entry to change
13F2:0136 BAC803        MOV     DX,03C8
13F2:0139 B00F          MOV     AL,0F
13F2:013B EE            OUT     DX,AL
--increment dx by 1, to point to the port to actually
--set a pallette entry
13F2:013C 42            INC     DX
13F2:013D 88D8          MOV     AL,BL
--output the new pallette entries
13F2:013F EE            OUT     DX,AL
13F2:0140 EE            OUT     DX,AL
13F2:0141 EE            OUT     DX,AL
--we need a slight pause so this is effect is visible, otherwise it
--just looks like ugly staticy squares, so save our registers
13F2:0142 50            PUSH    AX
13F2:0143 53            PUSH    BX
13F2:0144 51            PUSH    CX
13F2:0145 52            PUSH    DX
--we want to test this by a single clock tick, 1/18 of a second, which
--we will use DI to compare against
13F2:0146 BF0100        MOV     DI,0001
--call int 1ah, function 00, which will get a clock tick, to initialize 
--out testing value and set in BX
13F2:0149 B400          MOV     AH,00
13F2:014B CD1A          INT     1A
13F2:014D 89D3          MOV     BX,DX
--cal again to get current value
13F2:014F CD1A          INT     1A
--subtract stored value from current value. If it is greater, than we have
--surpassed our delay time, and we need to keep moving, otherwise, test again
13F2:0151 29DA          SUB     DX,BX
13F2:0153 39D7          CMP     DI,DX
13F2:0155 77F8          JA      014F
--restore registers
13F2:0157 5A            POP     DX
13F2:0158 59            POP     CX
13F2:0159 5B            POP     BX
13F2:015A 58            POP     AX
--loop back to palette shift beginning
13F2:015B E2D7          LOOP    0134
--do the same thing as the palette shift above, but in reverse order, ramping
--the values up
13F2:015D B93F00        MOV     CX,003F
13F2:0160 FEC3          INC     BL
13F2:0162 BAC803        MOV     DX,03C8
13F2:0165 B00F          MOV     AL,0F
13F2:0167 EE            OUT     DX,AL
13F2:0168 42            INC     DX
13F2:0169 88D8          MOV     AL,BL
13F2:016B EE            OUT     DX,AL
13F2:016C EE            OUT     DX,AL
13F2:016D EE            OUT     DX,AL
13F2:016E 50            PUSH    AX
13F2:016F 53            PUSH    BX
13F2:0170 51            PUSH    CX
13F2:0171 52            PUSH    DX
13F2:0172 BF0100        MOV     DI,0001
13F2:0175 B400          MOV     AH,00
13F2:0177 CD1A          INT     1A
13F2:0179 89D3          MOV     BX,DX
13F2:017B CD1A          INT     1A
13F2:017D 29DA          SUB     DX,BX
13F2:017F 39D7          CMP     DI,DX
13F2:0181 77F8          JA      017B
13F2:0183 5A            POP     DX
13F2:0184 59            POP     CX
13F2:0185 5B            POP     BX
13F2:0186 58            POP     AX
13F2:0187 E2D7          LOOP    0160
--check if there is a key press. If not, keep going, otherwise, stop
--running the program. 
13F2:0189 B406          MOV     AH,06
13F2:018B B2FF          MOV     DL,FF
13F2:018D CD21          INT     21
13F2:018F 749E          JZ      012F
--reset video mode
13F2:0191 B80300        MOV     AX,0003
13F2:0194 CD10          INT     10
--return to DOS
13F2:0196 B8004C        MOV     AX,4C00
13F2:0199 CD21          INT     21