I included the link to OpenOCD. That is the best place to start. There are books and papers out there but both are crazy expensive.
JTAG is a higher level "language" of sorts besides a physical layer standard. There is a limited command set that each IC has to have, and there are additional commands unique to the IC.
If you plan to JTAG, learn about JTAG in as much detail as you can. There are many places online you can pick up pieces parts and figure out how it works.
Read the BSDL library file on your chip to understand what commands it can do, and how each pin will respond. You can also review the datasheet for the IC at several places for further information. Most of the embedded devices, including cell phones use an ARM cored processor, so reading up on ARM is a plus.
JTAG is really just one of the more popular serial bus solutions (besides SPI and I2C). It uses wires TDI, TDO, TCK, TMS, and sometimes TRST. Data out on falling edge of TCK, and data read on rising edge.
Think of RS232 on steroids (in a very distant fashion). Early JTAG was not through "riff boxes", simply wired directly to the LPT. That can still be done in a pinch (pins 1, 2, 14, 16, and 17). I would start there, if you want to do things "manually". Caveat - voltage does matter. Embedded systems have various voltage levels for their signaling, not just TTL. 1.8, 2.5, 3.3V. I rarely see 5V nowadays.
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