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Lesson 2 - PC Assembly Language: ──────────────────────────────- GENERAL DISCUSSION ABOUT PC MEMORY: There are fundamentally two different kinds of memory chips. 1. RAM (Random Access Memory) which you may write to and read from. 2. ROM (Read Only Memory) which as the name implies may only be read from. Even though a segment consists of 65,535 bytes of memory (counting from zero), it is conventionally called 64K (K = 1000) memory. Your PC may address 16 segments of memory. For the time being we will ignore extended and expanded memory. These 16 decimal segments are numbered in hex from 0000h through f000h. The first ten segments from 0000h through the top of 9000h are all dynamic RAM that may be written to and read from. Your program and its data may be located anywhere in this region as long as it is above the computer's operating system which is loaded during boot up and is usually located in segment zero just above the 1024 byte interrupt table which occupies the first 1024 bytes of segment zero. The memory in segment a000h is video RAM devoted to graphics video. The upper half of segment b000h is video RAM devoted to text mode video. The rest of the segments, c000h through f000h, are usually occupied with various chunks of ROM that contain more of the PC's operating system including ASCII and extended ASCII character sets in different sizes, plus various chunks of RAM in this area that vary according to the PC's manufacturer and video board installed. TEXT MODE VIDEO MEMORY: ────────────────────── Text mode video RAM memory occupies segment b000h from b800h up through b8ffh. Each text mode video page uses 4000 bytes of video RAM when in normal 25 lines per page with 80 characters per line mode. As such up to 4 different text pages may be accommodated along with their VGA 4096 byte character sets. See TUTOR5.COM for a character set display. Exercising KISS restraint, we will use only the first text page. A text mode page in video memory alternates each 80 character displayed line with a data byte (the ASCII character displayed) with an attribute byte. The attribute byte tells the computer which color to display for each of its data bytes in the foreground, the background, normal intensity or high intensity and with to blink or not blink. So, each 80 character text mode video line occupies 160 bytes of video memory. One byte for the character + 1 byte for the attribute times 80 characters = 160 bytes per line. The character byte is first, followed by its attribute byte. Once the program sets the es segment to b800h and the video page to page zero the beginning and ending line addresses of a typical text video page are shown below in decimal and hex. You will recall that each byte has eight bits counting from zero = 76543210. The little diagram below shows how the arrangement of these bits determines the foreground color, background color, intensity and whether the data byte is blinking or not blinking. The cathode ray tube in your video display has 3 masks that the electron beam sweeps over. Each mask has a hole over a red, green and blue phosphor that illuminates when the beam strikes it. By combining these 3 colors and varying the intensity of the electron beam, the computer can create 16, 256 or even more colors depending upon the video card installed in your computer. The R G B letters below represent red, green and blue. line decimal hex begin end hex decimal line -- ---- --- --- ---- -- 0 0000 __ 000 09E __ 0158 0 1 0160 __ 0A0 31E __ 0318 1 2 0320 __ 140 1DE __ 0478 2 3 0480 __ 1E0 27E __ 0638 3 4 0640 __ 280 31E __ 0798 4 5 0800 __ 320 3BF __ 0958 5 6 0960 __ 3C0 45E __ 1118 6 7 1120 __ 460 attribute byte 4FE __ 1278 7 8 1280 __ 500 50E __ 1438 8 9 1440 __ 5A0 bit no. 7 6 5 4 3 2 1 0 63E __ 1598 9 10 1600 __ 640 ┌───┬───────┬───┬───────┐ 6DE __ 1758 10 11 1760 __ 6E0 │ BL│ R G B │ I │ R G B │ 77E __ 1938 11 12 1920 __ 780 └─┬─┴───┬───┴─┼─┴───┬───┘ 81E __ 2078 12 13 2080 __ 820 blink bit─┘ background│ foreground 8BE __ 2138 13 14 2140 __ 8C0 0 = off │ 95E __ 2398 14 15 2400 __ 960 1 = on └─ intensity bit 9FE __ 2558 15 16 2560 __ A00 0 = normal A9E __ 2718 16 17 2720 __ AA0 1 = high B3E __ 2788 17 18 2880 __ B40 BDE __ 3038 18 19 3040 __ BE0 C7E __ 3198 19 20 3200 __ C80 D1E __ 3358 20 21 3360 __ D20 DBE __ 3518 21 22 3520 __ DC0 E5E __ 3678 22 23 3680 __ E60 EFE __ 3838 23 24 3840 __ F00 F9E __ 3998 24 Here is a table that displays the R G B bit values in the attribute byte that display the eight fundamental colors in normal or high intensity. ┌─────────────────────────────────────────────┐ │ Foreground/Background Color Codes │ ├────────┬───────────┬────────────────────────┤ ┤ │ R G B │ Color │ │ ├────────┼───────────┤ │ │ 0 0 0 │ Black │ Setting the "I" - bit │ │ 0 0 1 │ Blue │ will result in eight │ │ 0 1 0 │ Green │ additional shades of │ │ 0 1 1 │ Cyan │ these eight colors for │ │ 1 0 0 │ Red │ the foreground color. │ │ 1 0 1 │ Magenta │ │ │ 1 1 0 │ Yellow ---> (or medium orange) │ │ 1 1 1 │ White │ │ └────────┴───────────┴────────────────────────┘ │ ┌────────────┬────────────┬───────────────────┐ │ Foreground │ Background │ Text Color │ ├────────────┼────────────┼───────────────────┤ │ 1 1 1 │ 0 0 0 │ normal display │ │ 0 0 0 │ 1 1 1 │ reverse video │ └────────────┴────────────┴───────────────────┘ All of Lesson 1 and Lesson 2 uses an attribute byte of 7 decimal = 00000111 binary which = medium white on a black background. Let us write a little source code program and then assemble and run it that illustrates how the attribute byte determines the data byte's color. Here is the source code for DEMO3.ASM. Use EDLIN.COM to write the source code and A86.COM to assemble it. Type edlin demo3.asm and press enter to load it. Then type 1P and press enter to display the first 23 lines of the source code. When ready to assemble your source code, type: A86 +LS DEMO3.ASM and press enter. The +L tells the A86.COM assembler to use 3 byte instructions for all unconditional jumps, rather than a jump shot instruction. This makes the code a tiny bit longer, but makes it much easier to trouble shoot. The S after the +L tells the program NOT to create a symbol table if the assembly is completed without errors. We DO NOT use the symbol table. DEMO3.ASM: ───────── 1: start: mov ds,cs ;data segment = code segment 2: mov ax,3 ;reset text mode = 3: int 10h ;clear the screen. 4: call cursof ;turn off the cursor 5: mov es,0b800h ;text mode video segment 6: mov ah,1 ;start with medium blue 7: mov di,>s0 ;list 7 medium colors 8: mov si,844 ;video location to start diz2 9: mov cx,7 ;display 7 lines 10: call diz2 ;display 11: mov ah,9 ;start bright blue on black 12: mov di,>s1 ;list 7 bright colors 13: mov si,1964 ;video location to start diz2 14: mov cx,7 ;display 7 lines 15: call diz2 ;display 16: mov ah,15 ;bright white 17: mov cx,1 ;display 1 line 18: mov di,>s2 ;exit message 19: mov si,0 ;display top left 20: call diz2 ;do it 21: mov ah,0 ;await key press 22: int 16h ;keyboard 23: cmp ah,1 ;Esc key 24: jz exit ;if so, go to exit 25: jmp start ;start all over 26: 27: diz2: mov al,[di] ;get character to display 28: inc di ;next one 29: cmp al,0 ;test for zero end of string 30: jz diz3 ;if so, go to diz3 31: mov es:[si],ax ;display ASCII char.+ attribute 32: add si,2 ;next video mem address 33: jmp diz2 ;continue 34: diz3: inc ah ;next color 35: add si,98 ;next video mem to display 36: loop diz2 ;display 7 lines bright colors 37: ret ;return to next after call 38: exit: mov ax,3 ;reset text mode video 39: int 10h ;also turns cursor ON 40: mov ax,4c00h ;exit instruction 41: int 21h ;return to DOS> prompt 42: 43: s0: 44: db 'This is medium blue on black',0 45: db 'This is medium green on black',0 46: db 'This is medium cyan on black',0 47: db 'This is medium red on black',0 48: db 'This is medium magenta on black',0 49: db 'This is medium orange on black',0 50: db 'This is medium white on black',0 51: s1: 52: db 'This is bright blue on black',0 53: db 'This is bright green on black',0 54: db 'This is bright cyan on black',0 55: db 'This is bright red on black',0 56: db 'This is bright magenta on black',0 57: db 'This is bright yellow on black',0 58: db 'This is bright white on black',0 59: s2: db 'Esc to exit',0 60: 61: curson: mov ah,1 ;set cursor type 62: mov cx,0607h ;cursor type 63: int 10h ;do it 64: mov dx,0 ;cursor position 65: jmp >c1 ;> = forward jump 66: cursof: mov dx,1900h ;cursor out of view on text 67:c1: mov ah,2 ;set cursor position 68: mov bh,0 ;page zero in text mode 69: int 10h ;do it 70: ret ;return to call+next instruct. When writing assembly language source code, the upper case (CASE) or lower case (case) makes no never mind, but hot shot programmers always use lower case exclusively. Until I got drug store reading glasses, I always used upper case because it was easier for me to read. This lesson's call diz2 in demo3.asm's lines 10 and 15 illustrates yet another way to display zero terminated ASCII strings. A zero terminated ASCII string is nothing more than a group of ASCII characters beginning with a db (define byte) instruction, then an apostrophe (') to tell the assembler that what follows are ASCII characters (decimal 32 through decimal 127). The next apostrophe at the end of the string tells the assembler that it the end of the ASCII characters. The next 0 tells the diz2 subroutine that it is all done displaying that line. The comma before the 0 tells the assembler that what follows should be treated as though it were on the next line with a db before it. As such, you can mix ASCII, decimal and hex on the same line if you follow these rules. It surely saves unnecessary line numbers. THE PC KEYBOARD: ──────────────- The PC keyboard and the operating system's decoding of its output is a pretty sophisticated series of subroutines using a number of different interrupts to handle the decoding. The interrupt we will use in these lessons is interrupt 16h. It is the most useful for our purposes and will handle most everything imaginable when used correctly. It has three fundamental functions. Number 1: ───────- mov ah,0 - int 16: this is the fundamental wait for a key to be pressed. It returns the value in the 16 bit ax register. The 16 bit ax register consists of two 8 bit registers. The al register for the low 8 bits and the ah register for the high 8 bits of ax. Thus, ah + al = the ax register. After mov ah,0 - int 16h is executed in your program it returns the values in hex in both the al register and ah register as illustrated below for the A key. All values are in hex. Key NoShift Shift Ctrl Alt ah-al ah-al ah-al ah-al A 1E/61 1E/41 1E/01 1E/00 We can see that register ah returns the scan code 1E when the letter A is pressed whether or not the shift, Ctrl or Alt key is pressed with it. A scan code is the value for the key pressed when the key is released on current PCs. Register al returns the ASCII value for the letter 'A' if the shift key is also pressed or the ASCII value for the letter 'a' if the shift key is not pressed. We need only compare register al with 61 hex or 41 hex to determine whether or not the 'a' or shift 'A' key is pressed. For our program to determine whether or not the Ctrl A or Alt A keys are pressed we must compare register ax with 1E01 hex or 1E00 hex. The table below illustrates the ax, ah and al register values returned by the mov ah,0 - int 16h function for ALL the keys on current/up to date, PC compatible computers. IBM PC KEYBOARD SCAN/ASCII CODES IN HEX RETURNED IN ax by: mov ah,0 - int 16H ────────────────────────────────────────────────────────────────────────────- Key NoShift Shift Ctrl Alt Key NoShift Shift Ctrl Alt ah-al ah-al ah-al ah-al ah-al ah-al ah-al ah-al A 1E/61 1E/41 1E/01 1E/00 F1 3B/00 54/00 5E/00 68/00 B 30/62 30/42 30/02 30/00 F2 3C/00 55/00 5F/00 69/00 C 2E/63 2E/43 2E/03 2E/00 F3 3D/00 56/00 60/00 6A/00 D 20/64 20/44 20/04 20/00 F4 3E/00 57/00 61/00 6B/00 E 12/65 12/45 12/05 12/00 F5 3F/00 58/00 62/00 6C/00 F 21/66 21/46 21/06 21/00 F6 40/00 59/00 63/00 6D/00 G 22/67 22/47 22/07 22/00 F7 41/00 5A/00 64/00 6E/00 H 23/68 23/48 23/08 23/00 F8 42/00 5B/00 65/00 6F/00 I 17/69 17/49 17/09 17/00 F9 43/00 5C/00 66/00 70/00 J 24/6A 24/4A 24/0A 24/00 F10 44/00 5D/00 67/00 71/00 K 25/6B 25/4B 25/0B 25/00 F11 85/00 87/00 89/00 8B/00 L 26/6C 26/4C 26/0C 26/00 F12 86/00 88/00 8A/00 8C/00 M 32/6D 32/4D 32/0D 32/00 N 31/6E 31/4E 31/0E 31/00 NUMERIC KEYPAD O 18/6F 18/4F 18/0F 18/00 P 19/70 19/50 19/10 19/00 Ins 0 52/00 52/30 Q 10/71 10/51 10/11 10/00 End 1 4F/00 4F/31 75/00 00/01 R 13/72 13/52 13/12 13/00 Dn Arrow 2 50/00 50/32 00/02 S 1F/73 1F/53 1F/13 1F/00 PgDn 3 51/00 51/33 76/00 00/03 T 14/74 14/54 14/14 14/00 <-- 4 4B/00 4B/34 73/00 00/04 U 16/75 16/55 16/15 16/00 5 4C/35 00/05 V 2F/76 2F/56 2F/16 2F/00 --> 6 4D/00 4D/36 74/00 00/06 W 11/77 11/57 11/17 11/00 Home 7 47/00 47/37 77/00 00/07 X 2D/78 2D/58 2D/18 2D/00 Up Arrow 8 48/00 48/38 00/08 Y 15/79 15/59 15/19 15/00 PgUp 9 49/00 49/39 84/00 00/09 Z 2C/7A 2C/5A 2C/1A 2C/00 - 4A/2D 4A/2D Space 39/20 39/20 39/20 39/20 + 4E/2B 4E/2B Esc 01/1B 01/1B 01/1B * 37/2A 37/2A 1 ! 02/31 02/21 78/00 / 35/2F 35/2F 2 @ 03/32 03/40 03/00 79/00 Del 53/00 53/2E 3 # 04/33 04/23 7A/00 Enter 1C/02 1C/02 1C/0A 4 $ 05/34 05/24 7B/00 5 % 06/35 06/25 7C/00 TOGGLE and SHIFT KEYS: 6 ^ 07/36 07/5E 07/1E 7D/A0 Read or Write MEM Location 0000:0417 7 & 08/37 08/26 7E/00 bit key when bit = 1 8 * 09/38 09/2A 7F/00 7 Insert insert mode on 9 ( 0A/39 0A/38 80/00 6 CapsLock capslock mode on 0 ) OB/30 0B/29 81/00 5 NumLock numlock mode on - _ 0C/2D 0C/5F 0C/1F 82/00 4 ScrollLock scroll lock mode on = + 0D/3D 0D/2B 83/00 3 Alt Shift key down [ { 1A/5B 1A/7B 1A/1B 2 Ctrl Shift key down ] } 1B/5D 1B/7D 1B/1D 1 Left Shift key down BakSp 0E/08 0E/08 0 Right Shift key down ; : 27/3B 27/3A ' " 28/27 28/22 Read or Write MEM Location 0000:0418 ` ~ 29/60 29/7E bit key when bit = 1 , < 33/2C 33/3C 7 Insert key down . > 34/2E 34/3E 6 CapsLock key down / ? 35/2F 35/3F 5 NumLock key down Enter 1C/0D 1C/0D 1C/0A 4 ScrollLock key down Tab 0F/09 0F/00 \ | 2B/5C 2B/7C 2B/1C (all above from IBM Technical Manual) TEXT MODE VIDEO LINE ADDRESSES: KEYPAD INPUT USEFUL ASCII CHARACTERS: 0-0000 6-0960 12-1920 18-2880 218 ┌ 194 ┬ ─ 196 ┐ 191 1-0160 7-1120 13-2080 19-3040 195 ├ 197 ┼ │ 179 ┤ 180 2-0320 8-1280 14-2240 20-3200 192 └ 193 ┴ ─ 196 ┘ 217 3-0480 9-1440 15-2400 21-3360 201 ╔ 203 ╦ ═ 205 ╗ 187 4-0640 10-1600 16-2560 22-3520 204 ╠ 206 ╬ ║ 186 ╣ 185 5-0800 11-1760 17-2720 23-3680 24-3840 200 ╚ 202 ╩ ═ 205 ╝ 188 The above table contains a great deal of information about the PC's keyboard output values. By all means print it out and place it in a single clear plastic cover sheet that you can keep at a handy place on your desk. I use it daily. On the other side of the plastic sheet print out and insert the: IBM - ASCII - DECIMAL - BINARY - HEX - TABLE This table was included as part of lesson 1. Number 2: ───────- mov ah,1 - int 16h: This function should be used to test the keyboard to see if ANY key has been pressed. It does not WAIT for a key to be pressed. The source code: mov ah,1 - int 16h - jz nopress will jump to nopress when no key is pressed. The following source code illustrates the correct way to use it. Line 5 does NOT wait since a key has been pressed. 1: mov ah,1 ;check to see 2: int 16h ;if key pressed. 3: jz nopress ;if not pressed go nopress 4: mov ah,0 ;unload keyboard buffer 5: int 16h ;it DOES NOT WAIT here 6: cmp al,61h ;test for 'a' key 7: jz gota ;if so, jump to gota 8: nopress: ;continue program here Number 3: ───────- mov ah,2 - int 16h: This function tests memory location 0000:0417 hex and returns its byte value in the al register. Memory Bit Key When bit = 1 ────── ─── ─── ──────────── 0000:0417h 7 Insert Insert mode 'on' 6 CapsLock CapsLock mode 'on' 5 NumLock NumLock mode 'on' 4 ScrollLock ScrollLock mode 'on' Use the test instructions as below to determine which bit is set. byte word bit no. 76543210 15 14 13 12 11 10 9 8 76543210 00000000 0 0 0 0 0 0 0 0 00000000 1. mov ah,2 2. int 16h 3. test al,10000000b ;bit 7 = 1 ? 4. jz insert_on 5. test al,01000000b ;bit 6 = 1 ? 6. jz capslock_on 7. test al,00100000b ;bit 5 = 1 ? 8. jz numlock_on 9. test al,00010000b ;bit 4 = 1 ? 10. jz scrolllock_on Note that we have illustrated yet another way to input a value in source code in addition to decimal and hex, b = binary. Sometimes it is easier to visualize the individual bits of a byte or word when using b = binary, rather using decimal or hex values. Your IBM-ASCII-DECIMAL-BINARY-HEX-TABLE reference sheet has all of these printed out for reference from zero through 255 decimal. Keep this sheet handy on your desk. I use it many times every day. On Friday night of this week, please take the little 20 question Quiz 2 and e-mail it to me tomorrow morning. Love, Grandpa