1. Set register $70 to the beginning of the ROM (EE00 0000 0000 0000) by means of SETH $70, EE00 instruction (its code is E0 70 EE00; see this link for operation details).
2. Find subroutine you need in the list and remember the displacement of its entry from the beginning of ROM (see entry parameter).
3. Define input parameters if required (see subroutines list for details). Usually E-MMI ROM use $71 for this aim.
4. Add GOI $6F, $70, <entry> instruction (9F 6F 70 <entry>) to your program.

The number of registers from $6F I have selected arbitrary. I strongly hope MMIX users will not want to call into requisition these registers. If this by mischance happens and somebody will need exactly these numbers, I can recompile E-MMI software ROM for him from any value he points.

Example:
To print $1 as integer value input the following code:

addr	code	mnemonic	action	comments
0	E0 70 EE00	SETH $70, EE00	$70 <== EE00	set the beginning ROM address
4	21 71 01 00	ADDI $71, 01, 0	$71 <== $1	move $1 to $71
8	9F 6F 70 10	GOI $6F, $70, 10	call $70+10, return - $6F	call ROM subroutine
C	00 000000	TRAP 0	exit to system

Note that ROM subroutines never modify $70 with ROM beginning address. So if you'll not also do this modification, you may set $70 only once in the beginning of your program.

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• What is E-MMI ROM?
• E-MMI ROM subroutines list
• ROM code

• Subroutine call (GO instruction)

• What is E-MMI?