MIT PDP-10 'Info' file converted to Hypertext 'html' format by Henry Baker

Byte instructions

In the PDP-10 a "byte" is some number of contiguous bits within one word. A byte pointer is a quantity (which occupies a whole word) which describes the location of a byte. There are three parts to the description of a byte: the word (i.e., address) in which the byte occurs, the position of the byte within the word, and the length of the byte.

A byte pointer has the following format:

 000000 000011 1 1 1111 112222222222333333
 012345 678901 2 3 4567 890123456789012345
 _________________________________________
|      |      | | |    |                  |
| POS  | SIZE |U|I| X  |        Y         |
|______|______|_|_|____|__________________|

POS is the byte position: the number of bits from the right end of the byte to the right end of the word.

SIZE is the byte size in bits.

The U field is ignored by the byte instructions.

The I, X and Y fields are used, just as in an instruction, to compute an effective address which specifies the location of the word containing the byte.

Here are the byte instructions.

LDB - Load byte. The contents of the effective address of the LDB instruction is interpreted as a byte pointer. The byte described there is loaded, right adjusted, into the AC. The rest of the AC is cleared.
DPB - Deposit byte. The contents of the effective address of the DPB instruction is interpreted as a byte pointer. The byte described there is deposited from the byte of the same size at the right end of the AC. AC and the remainder of the word into which the byte is deposited are left unchanged.
IBP - Increment byte pointer. The purpose of this instruction is to advance a byte pointer to point at the next consecutive byte. The AC field must be zero. The contents of the effective address are fetched. The POS field is changed by subtracting the size field from it. If the result of the subtraction is greater than or equal to zero, store the difference in the POS field. If the difference is negative, add 1 to the Y field (in the KA10 and PDP-6 if Y contains 777777 then this will carry into the X field; in the KI10 and KL10 the carry out is suppressed) and set POS field to 44-SIZE (44 is octal). The effect of this is to modify the byte pointer to address the next byte (of the same size) that follows the byte addressed by the original pointer.
ILDB - Increment and Load Byte. Increment the byte pointer contained at the effective address. Then perform a LDB function using the updated byte pointer.
IDPB - Increment and Deposit Byte. Increment the byte pointer contained at the effective address. Then perform a DPB function using the updated byte pointer.

Text strings are typically stored using seven-bit bytes, five per word. ILDB and IDPB can then be used to step through a string. The byte pointer should be initialized to 440700,,<address of string>. Then the first ILDB will increment it to point at the first character of the string.