18-bit SRAM

The Eval board has 1 Mw of 16-bit SRAM. That makes it awkward to store 18-bit instructions. This code packs and unpacks 18-bit data into 16-bit memory. It is surprisingly easy to do. Words are left-justified in a and right-justified in T, then T is right-shifted into a by +*.

Block 1158 is loaded into node 108, next to node 008 which is the SRAM controller.

9/8

Converts a count of 18-bit words to a count of 16-bit words. Multiples by 9/8.

Send the read/write command to 008
Add 1 to the ether 0-based count to get true count
Add 1/8 to that
Subtract 1 back to 0-based count
Send to 008
Send address to 008
Divide original count, which must be a multiple of 8 (-1), by 8

rd

Reads from memory 9 16-bit words and sends 8 18-bit ones.

Read first word
Loop 8 times
- Shift word left 2 bits
- Read next word, storing first in a. Low-order bits are 0
- Get loop index and or with 7. This counts up from 0
- Save on return for 2nd unext
- Use unext to shift T and a right. The +* instruction won't add register S to T
- Send 18-bit a
- Shift T back left, clearing low-order bits

+read

Analogous to +read in code for 008. Takes count and address and returns 18-bit words.

Pops ether focus into register b as where data should go
5617: Tell 008 to read count and call address 17
9/8 returns 0-based count /8. This is the number of 9-word packs to process

wr

Receives 8 18-bit words and writes 9 16-bit ones to memory. The inverse of rd.

Set T and S to zero
Loop 8 times:
- Save left-justified copy of previous word in a
- Get next word and copy both S (0) and it
- Retrieve loop index
- Shift T and a right. +* may add S to T, but S is 0
- Retrieve left-justified a, right-justify and write (sign bits ignored)
Write last word

+write

Analogous to +write in code for 008. Takes count and address and writes 18-bit words.

Pops ether focus into register b as to where data comes from
5625: Tell 008 to read count and call address 25
9/8 returns count of packs

init

Sets a and awaits instruction with 4-port read.