Step by Step Example: program to realize

Step by Step Example

Program to realize

Problem specification

Two integer numbers: A and B are stored in memory cells with addresses 30 and 38. Our program must compare these values and print to screen the results. For example, if A=3 and B=6 the output must be

compare result: A<B

The solution

Our program extracts A to register $1, B to $2 and then compares them. MMIX instruction CMP has a remarkable feature to return serial values -1, 0 and 1 when the result is <0, =0 and >0 respectively. So we just have to add constant 3D to this result in order to get the code of required symbol of inequality: '<', '=' or '>'.

To print the results of our program to console we'll use a special system input/output TRAP with 05 first parameter. The second operand of such instruction in E-MMI defines the number of input/output function. In discussing problem I used two kinds of output functions: 02 - print symbol to console and 09 - print string to console. The last parameter of TRAP 5 instruction is a register number that specify symbol or string address.

Described above organization of output is the specific feature of E-MMI project.

address code mnemonics operation comments

0 E3 00 0040 SETL   $00, 0040 $0 <== 40 set $0 equal to address of the text

4 00 0509 00 TRAP   5/09, $00 print string call system function: print string

8 E3 00 0030 SETL   $00, 0030 $0 <== 30 set $0 equal to A addres

C 8D 01 00 00 LDOI   $01, $00 00 $1 <== ($0) get 8-byte integer A value to $1

10 8D 02 00 08 LDOI   $02, $00 08 $2 <== ($0+8) get 8-byte integer B value to $2

14 30 00 01 02 CMP   $00, $01 $02 $0 <== $1 cmp $2 $0 = -1 if $1<$2, 0 if A=B, 1 if $1>$2

18 21 00 00 3D ADDI   $00, $00 3D $0 <== $0 + 3D ('=') $0 = '<' if $1<$2, '=' if A=B, '>' if $1>$2

1C 00 0502 00 TRAP   5/09, $00 print symbol call system function: print symbol

20 E3 00 0042 SETL   $00, 0042 $0 <== 42 ('B') put code of symbol 'B' to $0

24 00 0502 00 TRAP   5/09, $00 print symbol call system function: print symbol

28 00 000000 TRAP   0 exit to system

2C (not used)

30 A

38 B

40
48
50 63 6F 6D 70 61 72 65 20
72 65 73 75 6C 74 3A 20
41 24 compare
result:
A$ text: 'compare result: A' and ending '$'

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Related topics:

E-MMI software