bugfix: "start vertical retrace" field is stored as minus 1, and is 10 bits long (prior code only worked with 9 out of 10 bits).
bugfix: "start vertical retrace" field is stored as minus 1, and is 10
bits long (prior code only worked with 9 out of 10 bits).

joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib