The final piece of the puzzle is file transfer and the plan is to use the era-appropriate Xmodem. A modern version has been developed for 8080 CP/M by Martin Eberhard (founder of Tesla). Only basic functionality is needed, so the early version 1.01 is being added to the ROM at a cost of only 2k bytes. 

It's going to take a few more weeks though. The plan is to buffer the data on the CP/M BIOS side and hand it off in chunks to the kernel. The CP/M process will yield on a kernel timer to allow time for the chuck to transfer (10 blocks per byte). This will allow the full duplex serial connection to perform concurrent transfers: One instance of CP/M can be downloading a file while another is uploading.

In the meantime there is a way to transfer files to CP/M via the monitor: The monitor has some kernel functions available to get and put records to the RAM disk. These take the following form:

KG[track][sector] [address]
> KG0000 0000

KP[track][sector] [address]
> KP0000 0000

There are a total of 63 tracks (00-3E) with 32 sectors each (00-1F). Each sector contains a 128-byte record that can be read (kernel get) with KG or written (kernel put) with KP. The records are read from, or written to, a 16-bit address in the monitor memory.

The CP/M A: drive directory is 2k and occupies the first 16 sectors (track 00, sector 00-0F). The kernel format command KF will create an empty directory record and copy it to these sectors. The following monitor commands can be used to copy the first 4 sectors of the directory and display then in memory location 0-1FF:

> KG 0
> KG1 80
> KG2 100
> KG3 180
> D 1FF

 A file can be created in CP/M  by quitting the monitor (enter Q) and using the save command:

a> save 4 test.txt

This will save the first 4 pages of the TPA in a 1k file called test.txt. The directory entry will show a single block number of 02 which corresponds to track 00, sectors 10-1F. A file can be transferred over the monitor serial connection as a sequence of hex digits (one per line) using the monitor load command. The following would load data (e.g. 1k) to address 100 to 4FF and then transfer it to track 00, sectors 10-1F.

> L100
0100: FF
... copy hex, end with Q

> KP10 100
> KP11 180
> KP12 200
> KP13 280
> KP14 300
> KP15 380
> KP16 400
> KP17 480

The data transfer will overwrite the contents of the 1k test.txt file on the CP/M A: drive. This isn't very user friendly, but it was a useful technique for testing and debugging the CP/M BIOS and kernel code.