Big Number instructions

These instructions operate on variable byte length values.

| Byte 1       | Byte 2            | Byte 3           | Byte 4 - x | Byte x + 1 - y |
|--------------|-------------------|------------------|------------|----------------|
|      |       | 7 6 5 4 | 3 2 1 0 | 7654 | 3 2 | 1 0 |            |                | 
|   4      6   | len1    | len2    |   S     M     N  | Op1 (src)  | Op2 (dest)     |

• S = Instruction Selector (0-9 are legal)
• M = Extended Addressing mode for operand 1 (0-3)
• N = Extended Addressing mode for operand 2 (0-3)
• len1 = length of operand 1 in bytes - 1, 0 is operand length 1, 1 is operand length 2 etc.
• len2 = length of operand 2 in bytes - 1, as for operand 1.

There are 10 instructions.

Selector	Opcode	Description
0	A	Adds two big numbers. Dest = Dest + Src
1	S	Subtracts two big numbers. Dest = Dest - Src
2	C	Set flags based on subtraction Dest - Src
3	ZAD	Assigns a big number. Dest = Src (0 + Src, Zero Add)
4	ZSU	Negates a big number. Dest = -Src (0 - Src, Zero Subtract)
5	M	Multiplies two big numbers (signed). Dest = Dest x Src. Zero results where one operand is negative are unreliable
6	D	Divides two big numbers (signed) using a non-restoring binary division algorithm. Dest = Dest / Src. Remainder is not stored
7	DRM	Divides two big numbers (signed). Dest = Dest / Src. Remainder is written to address pointed to by A. Link flag is set for zero remainder
8	CTB	Base string to Big number. Base can be from 2 to 17. Operand 1 is src, Operand 2 is destination. Base in length field of Operand 1 (field holds Base - 2). Operand 1 length is provided in AL
9	CFB	Big number to Base string. Base can be from 2 to 17. Operand 1 is destination, Operand 2 is source. Base is length field of Operand 1 (field holds Base - 2). Operand 1 length is provided in AL. BL provides padding byte. AW holds address of result following operation. Result is formatted, @ in destination will be replaced by result. # in destination is padding marker, result will be zero filled until marker, from marker on result will be filled with padding byte. Other bytes in destination are left as is.

CTB and CFB Length Byte

| Byte 2            |
|-------------------|
| 7 6 5 4 | 3 2 1 0 |
| Base    | len2    |

• Base holds the conversion base - 2. i.e., Base 2 is stored as zero, Base 17 (!) is stored as F
• len2 is the length of operand 2 in bytes, minus 1. For CTB, this is the destination. For CFB, this is the source
• The length of operand 1 is passed in AL, minus 1. For CTB, this is the source string. For CFB, this is the destination string.